When the call came to say that I was to be photographed for a special exhibition at the National Portrait Gallery, I started to grapple with the fundamental question of mortality. A good portrait, I decided, is a rare opportunity to leave a legacy. It should show not just superficial appearances, but one's hopes, dreams and disappointments - in my case 20 years at the helm of the mental health charity, Sane. I also wanted to resolve the conflicts in my own life, as a former investigative reporter married to a Polish count. A portrait should show the inner essence of a person, I thought. Until, of course, I realised it is rather difficult to show one's soul in a photograph.

I looked in the mirror and got practical. I didn't want my portrait to look anything like my mirror image, like the Thomas Hardy poem:

"I look into my glass,

And view my wasting skin,

And say, "Would God it came to pass,

My heart had shrunk as thin!"

The last thing I wanted was to resemble me at all. Everyone my age feels that way.

How then to live up to a recent description of myself in a magazine as a "romantic crusader"? What would I wear? Could I ever look like a Pre-Raphaelite woman, my absolute ideal, with auburn curls, medieval robes and a luscious figure? I knew I didn't want to look dull: mental health is all too often associated with the drab and the dark.

Celebrities go to remarkable lengths to look their best and, while I don't see myself as a celebrity, I decided I might as well go for a bit of DIY self-improvement. My first port of call was the interior designer Nicky Haslam. We had dinner and he gave me the telephone number of a plastic surgeon. Discussions took place - quite serious ones, too - but with only three weeks until portrait day, or P-day as I came to call it, we questioned whether the bruises would disappear in time.

After deciding against surgery, I read that culture minister Tessa Jowell had managed to keep her looks, despite her various travails, by having oxygen facials. I resolved to give it a go: what's good enough for Tessa is good enough for me.

I had had only one facial in my life and it left my face looking like a scraped strawberry, so I was somewhat nervous. After stripping to my waist and having my solar plexus massaged while listening to wind-chime Muzak for an hour, my nervousness was replaced with impatience. I only wanted my face improved, not a spiritual transformation. It was fine, though, and afterwards I went to a costume shop on Shaftesbury Avenue and bought a purple satin cloak.

Three days before the shoot, I borrowed an expensive evening dress from Selfridges. Now all that was left was my much-exposed neckline: diamonds were called for, clearly. I rushed to Willie Nagle in Hatton Garden to find a necklace and earrings, which to my horror were valued at £50,000. They were on loan and obviously I couldn't afford to lose them, so for two nights I slept wearing these strings of wonderful shining light. I went to the local hairdresser and had a titian streak put in my hair.

It was left to me to decide where I wanted to be photographed, and what image of myself I wanted to project. The only rule was that the sitting should be indoors, and that the location should have some relevance to my work. In the end we settled for the Prince of Wales International Centre for Sane Research in Oxford, an inspiring blend of neo-classical architecture and gleaming laboratories, with an atrium filled (on my insistence) with exotic trees and my mother's grand piano.

P-day. Before the photographer, Julia Fullerton-Batten, arrived, we arranged the mis en scène: stems of roses, an open piano with Chopin sheet-music on top - a reference to my husband's family, in whose home the composer was born. Julia arrived with a whole band of lighting technicians and took four hours to set up. Meanwhile I waited nervously in the wings.

The trees and everything else I'd arranged so carefully were moved. I felt denuded. Instead, to fulfil the romantic brief, Julia produced a smoke machine. The Polaroids showed a phantom in a purple hood, a cross between the Scottish Widow and Countess Dracula emerging from the mist. Eight hours later, it was over.

Some weeks later I had a sneak preview. At first I didn't like it at all. I was alarmed to see myself marooned at a piano in an empty room. In the end, though, I think Julia was right not to let me have my way. The result is an arresting compromise between the minimalist, austere vision of a photographer - where all the action is offstage - and the insistence of the subject that she be surrounded with Pre-Raphaelite garlands and romantic drapes. As it is, the latter has been reduced to a crumpled carpet on the piano, and a vase of roses demoted to the floor. I might have preferred my original plan, but Julia has captured the loneliness of the long distance campaigner for an unfashionable cause, and, above all, the isolation felt by those who, through mental illness, have felt abandoned in a surreal and empty world.

· Marjorie Wallace was talking to Helen Pidd.

· A Picture of Health is at at the National Portrait Gallery, London WC2. Details: 020-7306 0055.