WE CREMATED Ian last month. We were only a small band - my mother, my two sisters, my brother-in-law and three neighbours. It was a deliberately modest service because Ian would have liked it that way. He never demanded any attention or wanted any fuss. Yet he will be missed by dozens more - relatives, friends, acquaintances, workmen to whom he gave the benefit of his advice, conmen who wheedled money out of him, and most of all, my mother, who looked after him most of his adult life.

Ian Thompson, or Air Vice Marshal Thompson as he sometimes called himself, was well known in Parliament Hill (a leafy suburb of north London, where he lived), because of his quirky ways, his deerstalker hat, corduroy trousers and unconventional conversation.

He had the mental age of a child of about eight, and like most boys of that age he was fascinated by cars, ambulances, fire engines and aeroplanes. If a car broke down outside my mother's house he always believed he knew exactly what was wrong with it. It was almost always 'a cracked distributor plate' or, just occasionally, the 'big end'.

Sometimes, he would stroll over to the frustrated motorist and offer his advice. It did not always go down terribly well. Indeed, I am sad to say that some drivers even cursed him, but it did not appear to ruffle Ian's feathers in the slightest.

He would chat to anyone who was willing to listen, and they would often naively assume that he was no different from any other elderly gent in the neighbourhood. Until, that is, he said goodbye, for he would often add: 'Might see you on the DoJ.' 'The DoJ?' the puzzled passer-by would ask. 'The Day of Judgment,' Ian would reply, chuckling to himself.

His story is remarkable, for it is one of real 'care in the community'. Everyone now favours the idea of looking after the mentally handicapped 'at home' whenever possible, but, in reality, it often does not happen. From Thursday, local authorities are meant to have enough cash to support the vulnerable in their own homes, or provide high-quality residential care for those who need it, but councils have already protested that the sums are not enough and the new system is heading for the rocks.

In Ian's case, he really was looked after 'in the community'. He was cared for by a family that was not his own but treated him with affection and respect (my grandparents for 30 years, then my mother for another 30), and by understanding neighbours who would occasionally employ him to do some gardening and reward him not just with a pound or two but with warmth and friendship.

How he came to live with my grandparents is a curious tale. He was born on 15 June, 1922, in Kenya, the son of the district commissioner, and was christened Ian Roland Broderick. His mother brought him to England when he was about four years old, but she died soon after.

His father, Charles Broderick Thompson, then arranged for him to board with a family in Kent. That arrangement ended when Ian was 11, so his father advertised in the Times for another family to take him in.

My grandfather, George Kirkpatrick, a Church of England rector in the Suffolk village of Copdock, and his wife, Lucy, volunteered for the task. They had plenty of space in their large rectory and very little money, so the weekly sum Charles Thompson was offering for Ian's care was a godsend. They already had a houseful of children - their own three daughters and a nephew and niece whose parents lived in India. One more would make little difference.

Ian had been teased at his school in Kent, but in Copdock he was accepted. My grand-parents had his squint surgically corrected, he put on weight and settled into family life. We do not know how soon after birth, his parents realised that he was handicapped (the result, we think, of some unidentified genetic defect), but it was certainly recognised by the time he arrived in the Kirkpatrick family. No one at the Copdock rectory thought it mattered much, and those days before the Second World War were probably the happiest days in his life. He did not go to school but was taught at home by my grandfather. He learnt to read and write but could never master sums, and even as an adult had no concept of money.

After the death of my grandfather in 1945, he lived quietly with my grandmother in Norfolk, helping with the gardening and domestic chores. His own father died in 1955, my grandmother in 1963. Without a second thought, my parents took him in, as they had planned to do, and as they had promised my grandmother they would. We were living in a three-bedroom flat, so did not have a great deal of space, but we managed.

I was 16 at the time, and sometimes embarrassed to be seen with Ian in case someone thought he was my father, which occasionally they did. As I grew older, that fear wore off. Three years later my parents moved into a large house, so space was no longer a problem.

Although in his youth his fantasy world revolved round the RAF (hence the Air Vice-Marshal title), as he grew older he identified more with a diesel lorry. He used to drive round the house, changing gear and turning an imaginary wheel. My mother entered the spirit of the game and would say: 'Change your wheels Ian, we are going out', or 'Have you emptied your sump? It's time for bed.' He would never go to sad events, such as funerals, in case he 'flooded his radiator'.

Ian lived happily with my parents and then, after my father died in 1990, with my mother. She was paid a regular sum to look after him (interest from his father's estate, the capital from which went to several charities on Ian's death), which reached about pounds 30 a week at the time of his death. He also received invalidity benefit for some of his adult life, after a friend told my mother he qualified for it, and, after the age of 65, his pension.

He was easy to look after, since he could wash and dress himself and even fix himself a simple meal (a boiled egg and toast) if he had to, but my parents could never leave him overnight. When they went on holiday, he would go and stay with one of my mother's sisters, both of whom also loved him dearly.

'He gave us far more than we ever gave him,' says my mother, Pauline. 'He was never any trouble. Even the way he died did not cause trouble.' He had a heart attack at teatime in the drawing room (at the age of 70), so he saved anyone the bother of carrying him down three flights of stairs from his room.

He got words and phrases slightly wrong. He and my mother were once passing Westminster Abbey when some unusual flag was flying. 'They must be expecting some deity,' he said. Perhaps they were.

We all flooded our radiators at his funeral. We are going to miss him dreadfully.

(Photograph omitted)