Blood is going to be its theme. I've made that decision long before I shall even begin writing the book. Blood in its metaphysical sense as the conductor of an inherited title, and blood as the transmitter of hereditary disease. Genes we'd say now, but not in the nineteenth century when Henry Nanther was born and grew up and achieved a kind of greatness, not then. It was blood then. Good blood, bad blood, blue blood, it's in the blood, in cold blood, blood and thunder, blood thicker than water, blood money, blood relations, flesh and blood, written in blood — the list of phrases is endless. How many of them am I going to find apply to my great-grandfather?

I'm not sure if I'd have liked him, and up till now it's been essential for me to like, or at least admire and respect, the subject of the biographies I write. Perhaps, this time, it's only going to be necessary for me to be interested in him. And that won't be difficult. It's only because I found out that he'd kept a mistress for nine years and, when his fiancée died, married her sister (giving her, incidentally, the same engagement ring) that I decided to write his life at all.

Of course I knew, we all knew, he'd been an eminent medical man, the acknowledged expert of his day on diseases of the blood and Physician-in-Ordinary to Queen Victoria. I knew that for his services Victoria had given him the peerage I've inherited, and that he took his seat in the House of Lords in 1896. But although he was distinguished in his day, an acquaintance of Darwin and mentioned as a friend in letters from, among others, T. H. Huxley and Sir Joseph Bazalgette, although he was the first doctor of medicine ever to receive a peerage — the great surgeon Joseph Lister got one a year later - as a biography candidate I was only keeping him in the back of my mind. In the front of my mind I had Lorenzo da Ponte, Mozart's librettist. Now there was an interesting history: unfrocked priest, political dissident, philanderer, storekeeper, distiller and professor of music at Columbia University. I could have got trips to Italy out of it and maybe Vienna, but reluctantly I had to give him up. I don't have enough musical knowledge. Then the letter came from my sister.

Our mother died last year. Sarah has had the job — it's always the women who get it, says my wife — of sorting out and disposing of or keeping her personal possessions. Among them was a letter from our great-aunt Clara to our grandfather. Sarah thought it would interest me. She even wrote, 'If you've given up the Marriage of Figaro man, why not Great-grandfather?' I've never seen any of Clara's letters before - why would I? - but I've a feeling she wrote a lot. My mother probably once had Sarah's task of sorting through her father-in-law's possessions when he came home from Venice to die, found the letter and simply neglected to throw it away.

It makes me feel a bit uneasy, disquieted and at the same time slightly excited, to notice the way Clara, his fourth and youngest daughter, refers to her father not as 'Father' or 'Dad' or 'Papa' but as 'Henry Nanther'. Odd, isn't it? Here is this maiden lady, to use my own father's expression for her, half-educated, who lived a quiet life in London, who never worked for her living and died at the age of ninety-nine, writing to her brother of their father as if he were some acquaintance she didn't much like. Her letter is dated early in 1966 and must have been sent to Venice. This is what she said:

You always speak of Henry Nanther as if he were some sort of pillar of society and high morality and all that sort of thing. In contrast to you, as you put it. I know you disliked him as much as the rest of us did with the exception of poor George. You will say that if he was a more or less absent father and a remote rather awful figure in the household, that was normal when we were children. But did you know he kept a mistress in a house in Primrose Hill for years and years? I am sure you did not know he was engaged to Mother's sister Eleanor, who was killed in the train. We had all heard the story of the train, but neither Mother nor Henry Nanther ever said he was engaged to her first, and then took up with Mother when she was dead. They kept that dark for reasons of their own, or reasons of his own. Henry Nanther did other monstrous, quite appalling things, but I don't feel like telling you about them in a letter. If you are interested you can ask me when you come home in August and we will have a good long talk. But, dear old Alex, you may not like what you hear. . .

Did they ever have that talk and did my grandfather learn what the monstrous things were? If he did he either didn't pass them on to my father or my father didn't pass them on to me. Before he could come home, he was dead. He died in June. But that letter was what fixed me. Sarah was right. I've been amassing Henry Nanther memorabilia ever since. Fortunately, he obviously wanted his 'life' to be written, and left behind him all the diaries, letters and works of his own he calculated would furnish suitable material. It's apparent, though, that he took care it included nothing that might show him in any light but that which shines on paragons. And of course there's not a sheet of paper, a photograph or a diary note among the lot to give a clue as to what Clara meant.

No one is at home but me. The house is empty, as it always is on weekdays at this time. Jude is at the publishing house in Fulham where she works and Lorraine, who cleans for us, left an hour ago. I am sitting in my study in Alma Square, working at what used to be our dining table: a large heavy mahogany affair, six feet long by three feet six wide, its surface marred by greyish blotches and black rings where Jude and I have put down hot plates, under the mistaken impression that the other one had put down the felt underlay before laying the table. Like most people we know, we have given up dinner parties, so I have 'commandeered' (Lorraine's word) the dining table and brought it in here.

Desks are never big enough. Desks are for chief executives in offices who have secretaries. On the dining table, as well as the computer and printer, are: the Shorter Oxford Dictionary and Roget's Thesaurus, a stack of (fairly useless) printouts from medical Web sites on the Internet, several mountain ranges of photocopied extracts from medical books, medical books themselves, Bulloch and Fildes' A Treasury of Human Inheritance, Henry's notebook, three box files of his correspondence, three books from the London Library, which, incidentally, I see are due back today, and a rather sketchy family tree, created by me and full of gaps. My own line, which begins with my grandfather Alexander and ends, for the time being, with my son Paul, is there in accurate detail. My grandfather's small brother George, who died aged eleven, is there, and so are his four sisters, Elizabeth, Mary, Helena and Clara, but I don't yet know the names of husbands, children and no doubt grandchildren, and I must hunt for them in the records.

As well as all this are the fifty-two leather-bound diaries, in various colours, designs and sizes, Henry kept from his twenty-first year until a year before his death, his own books, photograph albums and some loose photographs. Letters about him and letters which mention him are on the table too.

He was obsessed with blood. Why? He wrote books about it but he wrote private memoranda too; curious essays that, presumably, no one else ever read in his lifetime. These are contained in a notebook with a cover of black watered silk. One of them - I have it here, it's the top one in the red box file but like all of them, undated - starts:

I have often asked myself why it should be red and I have asked others. Among the answers given to me has been. 'Because God made it so.' If I had never seen it but knew of its existence, its presence and function in the human body, I would have expected it to be brown, a light yellowish-brown. But it is red, the pure scarlet of the poppies that grow in the cornfields and which I remember from my boyhood. One of my children asked me, as children will, what might be my favourite colour. I had no hesitation in replying to her: red. I cannot recollect that I paused to cogitate, to give any consideration to the matter, though I had never previously thought about it. The word 'red' sprang naturally to my lips and, as I uttered it, I knew I was speaking perfect truth. Red in my favourite colour. No one knows why blood is red, although its composition is of course known and pigment of this colour is present in it. To me a splash of blood is beautiful and I profoundly lack understanding of those who flinch or even faint at the sight of it.

Not in any way incriminating, though, is it?
A royal doctor who happened to be given a barony would hardly be a suitable subject for a biography unless he was otherwise interesting. He made one important discovery in his own field and thus contributed to the sum of medical knowledge, but he seems to have cured no one; I doubt if he even alleviated pain or would have wished to do so. Is that where the interest lies? Perhaps, rather, what fascinates is not only this obsession with blood but the mysteries of anomalies the biographer comes upon in every decade of Henry Nanther's life.

I make myself a cheese sandwich and eat a tomato with it. If I intend to have a look at the house in Hamilton Terrace and call in at the London Library on my way, I haven't much time. The House sits at 2.30 and I remember as I'm finishing my sandwich that I have the third starred question to ask.

This house that Jude and I live in, though far from an ancestral home, was my father's and my grandfather's but never Henry's. His was a huge stucco mansion on the other side of Abbey Road, next door but two or three to the much prettier house of Joseph Bazalgette who built London's drains and river embankments and not far from Lawrence Alma Tadema's studio. It's a mild pleasant sort of day, though so early in the year. I come down into Hamilton Terrace by way of Circus Road and pause on the other side of the street to take a look at Ainsworth House, the name he gave it. Having at one time been divided into flats, it has returned to its single owner-occupier status, the owner-occupier being a multi-millionaire property developer called Barry Dreadnought. Since buying it for three million, he has had the front garden paved with two square boxes inserted as flowerbeds and two enormous white urns each containing one of those spiky red palms. A covered way with plain glass sides and a stained-glass peaked roof runs from the gates to the front door.

I have never been inside. I wrote to Dreadnought last year asking if I could have a look at the room I calculate was Henry's study but he didn't answer, even though my letter was written on House of Lords paper. Will we still be able to use the headed paper after they've abolished us? I suppose not. I've never thought of that before and it slightly depresses me. If they won't let us keep club rights and computers they're certainly not going to let us have our paper.

The two sash windows on the second floor on the left indicate Henry's study. Or so I've guessed. There's something about the house - there was long before Dreadnought came - that troubles me, something unpleasant, I don't know what it is. Of course it's ugly, the worst kind of Victorian, but it's not that which makes me uneasy. I suppose it's that I sense there was suffering and misery within those walls while Henry and his family inhabited it, though I've no real reason to think there was, only a suspicion. As far as I know, Henry was happily married and, in spite of what Clara says, a good father by Victorian standards. I suppose I'm seeking inspiration when I come here to look at Ainsworth House, as I do every so often. Seeking perhaps answers to questions I, as Henry's biographer, ought to be able to answer but can't yet. Just the same, I wouldn't live in that house for all its owner's cash.

A woman stares at me out of Henry's study window. It's the dark unhappy face of one of those domestic slaves who look after other people's children and send money home for the care of their own. But no, I'm imagining that. Why should she be any different from Lorraine? The millionaire shouldn't be condemned out of hand just because he didn't answer my letter.

I take the Jubilee Line from St John's Wood and get off at Green Park, which isn't far from St James's Square where the London Library is. The rest of the way is on foot, through the park and over the bridge, and though I've walked this way a thousand times, I always stop for a second on the bridge to look at the view of Whitehall and Horse Guards and the Foreign Office: water and trees and majestic buildings, and the pelicans on their island. At this time of the year the great influx of tourists hasn't yet begun. It's an ordinary walk to the Palace of Westminster, not a battle through dawdling crowds with cameras as it sometimes is. Outside the Peers' Entrance, Richard Coeur de Lion sits on his horse, his sword arm and sword upraised. I always give him a glance and wonder what it was like to go on a crusade, when the rabble of peasant soldiers of Christ thought it quite permissible, indeed praiseworthy, to kill infidel women and their babies and roast them for dinner. The doorkeeper says, 'Good morning, my Lord,' though it is twenty-five past two, but, as peers should know when they've been here five minutes, it's morning in the House of Lords until prayers have been said.

I hang my raincoat on the peg that says 'Lord Nanther', which is next door but two to the Duke of Norfolk's, go up the staircase to the Principal Floor and in the Printed Paper Office I pick up an order paper and the list of amendments to the House of Lords Bill, and there is my starred question: 'What, in the opinion of Her Majesty's Government, is the likelihood of the Jubilee Line extension being completed in time to offer public access to the Millennium Dome by I January 2000?'

Not being fond of bishops, I stay out of the Chamber until after prayers. It's always a bishop that says them — there are twenty-four in here and two archbishops, and each does a week-long stint — but these days few of them use the fluting tone associated with the High Church of my father's youth. I go into the Chamber as part of the great influx from the Peers' Lobby and take my usual seat in the third row from the front of the cross-benches on the spiritual site. Not strictly the cross-benches at all, they are in the middle, parallel to the clerks' table and the throne, but an extension of the government area behind their bit of the front bench where the privy counsellors sit. Lord Callaghan and Lord Healey are often seated there but not today. My grandfather sat on the cross-benches on the rare occasions he attended, describing himself as independent and Bohemian. My father and Henry were staunchly right-wing, both of them die-hard Conservatives.

When I first entered this chamber at the age of eleven and sat on the steps of the throne as my father's heir (the Hon. Martin Nanther) I thought the place very ugly, its Gothic ridiculous, its colours crude, especially the kingfisher-blue carpet and blood-red leather benches. The gilding of the throne, almost too dazzling to look at, reminded me of a stage set in the Aladdin pantomime I'd seen at Christmas. Thirty-five years ago Gothic was still unfashionable, taken for granted as being in bad taste. I was particularly scornful of the stained-glass windows, uncompromisingly red, green, blue and yellow. But I was still young enough to like the carved figures of lion and unicorn which serve as finials on the posts at the bar. Now I feel differently, if I don't quite go along with whoever it was the other day that described the Palace of Westminster as the most beautiful building in London. It is beautiful and I shall miss it when I'm gone. I shall miss giving the unicorn a pat on his polished head when I pass through the gate in the brass barrier we call 'the bar', bow in the general direction of the throne and the Cloth of Estate (non-existent, spiritual, a space only, marking the position which the Queen, if present, would occupy) and make my way up the steps to my place. The Chamber is full, for today is the first day of the second reading of the House of Lords Bill. First reading is, of course, a mere formality, so the second reading is very important and feelings will run high. Quite a lot of hereditary peers acknowledge that their day is done, that a man or woman - there are a few women hereditaries - should not have a right to make the country's laws just because an ancestor helped the king in war or an ancestress slept with him. It's not this which most of them will dispute, but the uncertainty surrounding what kind of a House will come after them, the brutal talk about 'getting rid of them' and the loss of their privileges to eat and drink and smoke in the House and use its library - in other words, their club rights.

But questions first. There's one about Railtrack and one about nuclear weapons and then comes mine. The Clerk of the Parliaments gets up and says, 'The Lord Nanther,' and I say, 'My Lords, I beg leave to ask the question standing in my name on the order paper,' but I don't ask it because it's printed for all to read on the Orders of the Day.

The minister says there's no question but that the Jubilee Line will be completed. I'm obliged to ask a supplementary question and this is something that makes a lot of peers sweat, lest the minister pre-empts them and they're left with an enquiry that's already been answered. Also you may write down your possible supplementaries as a mnemonic but not read them aloud. When I first came in, I got into a muddle over this and Conservatives began chanting, 'Reading, reading!' I resolved never to ask another starred question but of course I did, and then another, and now it doesn't worry me much. Very soon it won't worry me at all but by then I shall have been banished.