TODAY, a plaque in honour of Clive Staples Lewis is being unveiled in Poets' Corner, Westminster Abbey. What would he think about joining the likes of Chaucer, Shakespeare, Keats, Blake, and Eliot? One more reliable guess than most comes from Douglas Gresham, who, as a young American boy, became Lewis's stepson in 1956.

"I think he should laugh, quite frankly, because the one thing Jack never succeeded at was as a poet."

Mr Gresham, who was visiting London to promote a collection of Lewis special editions from the literary estate of which he is steward, says that, from early on, the Belfast-born scholar wanted to be the next great lyrical poet. His first poetry collection, Spirits in Bondage, ignited no critical reaction. Neither did a second volume, Dymer, published six years later. Probably just as well, Mr Gresham says.

"He was guided away from lyric poetry to prose. But his prose is some of the finest prose poetry you'll ever read. He was far better at it. So the fact that he's going to be honoured in Poets' Corner is a little bit ironic. I think Jack would find that amusing."

Mr Gresham, like everyone else, can only speculate on some aspects of Lewis's life, but in other areas he is a reliable witness. When his mother, Joy Davidman, married Lewis, he and his older brother, David, became his adopted sons.

When she died of cancer in 1960, it was Lewis who raised the boys until his own death, three years later. Divorced two years previously from her husband, the writer William Gresham, Joy had moved with her boys to England in 1953, after getting to know Lewis during an earlier stay in Oxford.

MR GRESHAM is a cheerful, eccentric figure in polo shirt, gold rings, and black leather riding boots. Now aged 68, he has been involved in several films of Lewis's books, and remembers those first days with a cinematic quality. The "little boy from upstate New York" knew all about Lewis from his mother, and had been reading The Lion, the Witch and the Wardrobe, and also King Arthur and the Knights of the Round Table, by Lewis's friend Roger Lancelyn Green. His eight-year-old head, he recalls, was "full of legends, with people in England riding on horses, wearing armour, carrying swords and longbows."

This demanded a quick rethink. "It all falls apart when you're invited to meet C. S. Lewis, the man who's on speaking terms with the great lion Aslan, and High King Peter of Narnia.

"What flies to mind is an image of a man wearing silver armour and carrying a sword, and probably wearing a coronet; and, of course, when we walked through the door of this very shabby house in Oxfordshire, in Headington, it was disappointing to meet a stooped, balding, professorial-looking gentleman with long nicotine-stained fingers and teeth, in the shabbiest clothes I think I've ever seen anybody wearing."

He remembers Lewis at that time as less portly than in later years - "an agile individual", happily spending time with the young brothers sawing up logs for the fire. "He was always involved with outside pursuits, and great fun to be with, full of energy, very fit." But then, he says, he did look like "a not-quite-retired farmer".

Lewis's academic reputation as a professor of medieval literature, and his prolific output obscured the way that laughter followed him around, Mr Gresham says. The boy's initial disappointment was "erased by his huge personality, the enjoyment of life that he bestowed on those around him. I quickly lost an illusion, and gained a very good friend, and later a very good stepfather."

A DECADE previously, in The Abolition of Man, Lewis had written: "I myself do not enjoy the society of small children. I recognise this as a defect in myself." And, despite his imaginative genius in entering a child's mind to conceive The Chronicles of Narnia, Lewis, Mr Gresham says, always felt that personally he did not relate well to children.

In a letter to his close friend Arthur Greaves, he described himself as shy in the presence of children. Mr Gresham thinks that he and his brother probably helped him to overcome that, and to "come to grips with the development of a child's mind over a long period of time.

"He describes me in a letter to someone as an absolute brick. Which is an old-fashioned term meaning a first-class chap . . . which is another old-fashioned term, now I come to think of it!"

It was an old-fashioned world that they met in and shared, and one of the recent criticisms of Lewis's work is its "golly goshness", to borrow a phrase of his recent biographer, Alister McGrath. But we are all products of our time, and Lewis, Mr Gresham says, was always a product of his Belfast upbringing, which, despite his growing fame, left him - and his bachelor brother, Warren (Warnie) - permanently anxious "that he was going to wind up in penury, impoverished".

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"Even when they were very comfortably off, they were in fear," Mr Gresham says. "They wouldn't spend a cent on their house or anything, until my mother forced them to. They were afraid my mother would open the books and say, 'Look, look, you've got the money to do this restoration, or renovation,' or whatever."

IN TIME, Joy, the one-time atheist and Communist and now a Christian believer, helped the brothers to relax. For a start, she redecorated The Kilns, the house they shared, which, Mr Gresham says, had been "an absolute shambles. The books and bookshelves were reputed to be holding the roof up. The walls were peeling plaster.

"I was lying on my bed once, and some angel told me to look at the ceiling, which I did, and I saw it start to fall."

When his mother's bone cancer went into remission, she decided that enough was enough. "She looked around, and said: 'This is ridiculous, I've come back to life, and this old house is going to come back to life, too.' She dragooned Warnie and Jack to get on to it, and completely restored the house, and it became a very comfortable place to live." Even though the brothers grumbled about the expense, he says, secretly "they loved it". Joy would show them the accounts and say: "Look, it's a drop in the bucket."

Haunted by anxiety about penury or not, Lewis was extremely generous, something often revealed only accidentally. Mr Gresham tells how an Egyptian man came up to Lewis, after a lecture, to recount how his government had cancelled funding for his studies at Oxford.

An envelope arrived under the man's door from a London legal firm containing a cheque, the first of several, which meant that he could finish his studies. Much later, he found out that Lewis was his secret benefactor.

Mr Gresham, a seasoned Lewis after-dinner speaker, has a fund of anecdotes to draw on, whatever the enquiry. On the theme of generosity, he recounts a time when Lewis was approached by a beggar for spare change, and emptied his pockets into the beggar's hands. "His friend turned to Lewis and said: 'Jack, you shouldn't have given that man all that money, he'll only spend it on drink.' And Jack said: 'But if I'd have kept it, I would have only spent it on drink.'"

After Joy's death, the brothers and the professor were thrown more closely together; but, Mr Gresham says, Lewis never tried to take the place of his father. When someone suggested that he should change his surname to Lewis, his stepfather counselled against it.

"Jack said: 'No I don't think it's a good idea.' And I said: 'Why not?' He said: 'Because that would not be honouring your father, as we are commanded to do in the Bible.' And he was using the word 'honour' in the old-fashioned, King James Bible sense, which doesn't mean pay respect to a parent, but to honour one's father in the sense that one admits to who his father is."

MR GRESHAM's life has been dominated by the Lewis legacy ever since the death of Warnie, when the estate transferred to him. His mother, father, and stepfather died within three years of each other, in his teens, but meeting Meredith, his wife-to-be, set him on a new course. He worked on her uncle's farm, studied at agricultural college, and emigrated to Tasmania, where he become a dairy farmer.

Unlike his older brother, the Lewis legacy has turned out to be his vocation. While Douglas and his family became Evangelical Christians, David converted to orthodox Judaism, and they now have little contact. "Our lives have just gone in different directions."

As both devoted stepson and guardian of the Lewis legacy, Mr Gresham is rarely critical of Lewis. The man who wrote about being good in public, he says, was always trying to be good in private.

"I've walked into the study sometimes, Jack would be sitting there, in silence, and I knew he was praying. I'd say, 'I'm sorry, Jack,' and he'd say, 'Oh no, don't worry, I was only praying,' as if it's something he did all the time, which he did. All of our thoughts should be some form of prayer. And Jack put that into practice."

Now living in Malta, most days Mr Gresham continues to choreograph the Lewis industry, from co-producing Hollywood versions of Narnia titles to overseeing new editions of the books. Mere Christianity alone has sold more than 150,000 copies in the past year. The books will remain in copyright for another 20 years, and Mr Gresham intends to remain the guardian of every new iteration.

Lewis's own favourite, of all his titles, was Till We Have Faces, Mr Gresham says. It is a retelling of the myth of Cupid and Psyche. Lewis believed it should have been published as a collaboration with Joy - but she would not let him. "She said all she'd done was teach him how to write from a feminine perspective, and how to write more like himself. When I read it, I can hear my mother's voice in places . . . there's a lot of my mother in that book."

And there is a lot of Lewis in Douglas Gresham. "I've never met a man before or since who lived Christianity as keenly as Jack did. He showed me how to go through life, minute by minute, from the time you wake up, to the time you go to sleep, being a Christian.

"Christianity, for him, was not what you said, it wasn't what you wrote, and it wasn't what you be- lieved. It was what you did, because of what you believed."