To be honest, I am not really the sort of chap who hankers after having his portrait painted. No doubt, I have a duty to posterity - though I hope my descendants will have better taste than to want pictures of me hanging in their houses. But I cannot escape the feeling that there is something uncomfortably self-regarding about being immortalised on canvas. Ironically, though, it seems to have become a habit.

In the early Nineties, Gail Lilley, the talented artist wife of former Tory minister Peter Lilley, announced that she wanted to paint me. Despite the limitations of her subject, she did a superb job.  Her study of me sitting in an armchair brooding over a newspaper adorns our dining room, where it has acted as an appetite suppressant for countless visitors.

Then, a couple of years ago, my wife met Susannah Fiennes, one of Britain's leading young artists, best known for accompanying the Prince of Wales on various overseas trips, capturing scenes and people from his foreign travels. My wife was immediately attracted by Susannah's work, and decided she wanted her to paint me to mark my 40th birthday. The idea of a monument to my middle age did not immediately appeal, but I was persuaded.  When I studied Susannah's work - whether landscapes, street scenes or portraits - I was struck by its superb quality. Also, there was a large vacant space on the wall of our main staircase, and even a picture of me would improve it. 

As well as having her work collected by the Prince of Wales, Susannah has built up a significant reputation in America, where she is now based.  Her last three portraits have been of a Venezuelan, a Chinese and a Filipino, all well-known New York businessmen, and her next will be of Amanda Foreman, author of the best-selling Georgiana, Duchess of Devonshire.

Steve Forbes, the former US presidential hopeful, is one of her patrons. A drawing by her of former Hong Kong governor Chris Patten, made when she accompanied Price Charles to the colony's handover in 1997, is in the National Portrait Gallery.  She has also drawn her cousins Ralph Fiennes and Sir Ranulph Fiennes.

In an age of instant digital images, it is easy to see why some might think of portrait-painting as an anachronism. However, the interpretation an artist makes of his or her subject, never mind the technical quality of the portraiture, is unique and can bring out the character of a subject and his place far more profoundly than more modern means an hope to do.  Having your portrait painted can also, as I found, be an art education in itself. Some perfectly good portraitists keep sittings to a minimum and work largely from photographs, usually more for the convenience of the sitter. Susannah took a different, much more old-fashioned approach.

For seven and a half days - to my relief I had a day off in the middle - she and I stood opposite each other while she scrutinised and evaluated every aspect of me, and the corner of our dining room in which I was standing.

I tried to keep the same position in varying degrees of agony: never having had to stand still for days on end, I had not appreciated just how incredibly difficult it was.  We had settled on a full-length portrait at the time I commissioned Susannah. This was partly because in my other portrait I am sitting down, but also for the more prosaic reason that a big picture was needed to fill the gap on the wall we had in mind.  Also, it had often struck me when in art galleries that the most satisfying portraits were of people standing up.

When she arrived on the first morning, Susannah wanted a north-facing room so she could paint in natural light without direct sunlight.  Our dining room, with a big Georgian window letting in the north light, was judged to be ideal. Then came the question of exactly what pose I should strike. While discussing this I leant on the fireplace, and that was that. The pose was said to look natural: and this was how I ended up standing for about seven hours a day for more than a week. In the mornings, I found I could manage 30 or 50 minutes before rigor mortis set in. I was taking my weight on my left leg, and once that had had enough, I started to sway from side to side, which the artist found most disconcerting.  In the afternoons when fatigue began to bite, 20 minutes was the best I could do.  During our short breaks, we would usually go to the back of the house, taking the canvas with us, and sit in a south-facing room examining what had been done.

At the start, the canvas was covered remarkably quickly with the unmistakable shape of the subject: but Susannah warned that subsequent progress would be slow. It was.  As a painter, she is obsessed with geometry; and she took the most extraordinary pains to ensure that my proportions were right.  I was constantly being sized up with lengths of paintbrush and told how many times my head went into my body, or my feet went into my legs.  When the brush-length was not an adequate gauge, she tied a heavy tube of oil paint to a piece of string and made a plumb line. She would then tell me where my centre of gravity was, something I and others had been curious about for years. Of course, this was not so easy to judge when I was swaying around like a drunk, but she overcame this difficulty remarkably well.

The other problem was how much of my surroundings she should include in the portrait.  A marble fireplace, a mantelpiece with some Dresden pots on it and a demi-lune table all competed at the edges of the composition.

First, a rather large Heffer dominated the canvas, with only the most peripheral details. Then, on about day three, Heffer shrunk, the old boy scrubbed out and repainted at about two-thirds his original size.  Susannah was concerned that although by shrinking me she had managed to include more of the furniture, there was also an expanse of carpet at the bottom and of wall at the top. She began to feel she had been distracted by the furniture when she should have been concentrating on me, a state of affairs most people would readily understand and sympathise with.

We had clearly reached a crisis. On the resumption after our day off, various drastic options were discussed. We could saw six inches off the top and bottom of the canvas. We could get a new canvas and start again. Or we could restore Heffer to his previous larger proportions.

Starting again was unthinkable, as much for me as for Susannah. Cutting down the canvas would rather defeat the original object of a full-length picture. So she settled for fewer furnishings and more Heffer, who was quickly enlarged to his previous dimensions. With the details established, the hours until we finished were mostly spent getting the colours right - a question of how the artist interprets them in that particular light. Two in particular proved taxing: the deep red of our dining room walls, which is allegedly the same colour as the inside of the stomach and so, supposedly, aids digestion; and the sludge bronze-green of my corduroy trousers. My bright red hair gave little trouble.

We had set ourselves a deadline of when the painting would stop. As the hour approached, Susannah finally accepted that she had captured the proportions and the colours as best she could. Certainly I, and other who saw the finish picture, could find no fault.

Her perfectionism knows no bounds, however. As I limped away from the fireplace for the last time, feeling utterly geriatric and mightily relieved, she said that "there is always more to be done".  I hoped she was joking. Later on, though, she said she was returning to England in May, and would come and have "another look at it". I don't want to appear uncooperative, but I fear I may have to be elsewhere.