"What a little ragamuffin he used to be!" said his Dad. "And look at his pot belly! He hasn't got that now!"

You can tell from the affectionate way he talks about David that Mr McCallum has always had a soft spot for his second son and that they are still very close.

"He used to love walking around with me, chatting about anything and everything around us," he said. Iain (his brother who is four years older) was a different type of boy altogether. He was thoughtful. He read a lot whereas David could never bother with books. David was more interested in watching and observing people and things. I think that is what gave him the ability to portray different types of people. He has an awareness to moods and personalities that you don't often find. And he can imitate every dialect. You should hear him doing the Liverpool one!"

Although the past of Russian-born Illya Kuryakin, the U.N.C.L.E. agent second in line to Napoleon Solo (but second to none in the hearts of millions of fans) is mysterious, the roots of David McCallum the actor are easily traceable. They are firmly bedded in a comfortable, middle-class background.

Mrs Dorothy McCallum is everything a Mum should be. She's warm and friendly, intelligent, a fantastically good cook, and one of those women who never lose their good looks. She did tell me how old she is but I've conveniently forgotten because to state her actual age would conjure up a very misleading picture. She looks about forty although, of course, that's impossible. She must be a lot older. Her hair is naturally curly and auburn coloured. She's slim and was wearing a lovely Chanel-type suite when I went to see her at her Hampstead home. The smell of freshly ground coffee hit me as soon as she opened the door. And with typical motherly concern she put a stool in front of the fireplace. "Sit there until you get warm," she said. She disappeared to the kitchen and returned seconds later with three hot cups of coffee and a plate of petit fours. Dad appeared waving a Stradivarius violin. "It's priceless," he said, by way of introduction.

At home David is always called David Keith to avoid confusion with his father. David senior looks much taller than junior and he wears Arthur Miller glasses that give him an academic air. Almost like Kuryakin disguised as a physicist! We talked about the new fireplace they're having to put in because they now live in a smokeless zone and I resisted the temptation to have a biscuit - Although Dad warned me I was missing something delicious if I didn't have one.

"You're slimming, I suppose," said Mum. "It's not the weather to slim." Mums never like slimmers!

Dad went to a chair by the window and fiddled with his priceless violin. "I'm fitting a chin strap on it," he said. He called it a Strady or something. No doubt when you're a first rate classical musician like him you can get away with calling a Stradivarius by a pet name.

"Show business is a world we are very familiar with through Dad being a musician," Mrs McCallum said, "but we have never known anything like David's success, of course. These young Americans have really taken to our David Keith, but being his mother I can't see what all the fuss is about. It's strange because he was so highbrow when he was young! It was my idea he should learn the oboe. I think people who are naturally musical should be able to express themselves, don't you? He had been playing very well but he wanted to be an actor. He always wanted to be one even when he was quite small and nothing was going to stand in the way. He is a very intelligent boy. I had always hoped he would go to university and follow an academic career. It was a disappointment to me when he went to R.A.D.A. because I wanted him to go on and study. He still reads a good deal but he doesn't get much time now.

"Iain was always the brainy one, and it's not always easy following an older brother. Iain was also extremely athletic. He was in the school rugby team very early on - one of the youngest I think. When David came along four years later they expected him to be equally athletic but he hadn't the brawn. He's wiry, like his father, but extremely strong. He looks very slight but he must be strong to do this U.N.C.L.E. part. I don't know how much professional help he gets but he must do a lot himself. I'm so used to him being knocked around it doesn't worry me any more. He has been at it so long, I'm afraid I've got used to it.

"Although he was not so good at athletics as Iain he still got his school blue in swimming when he was at school. It was sometime after the war. Schools had not had time to do things because of the war but then they opened a new swimming baths and everybody suddenly learned to swim - and he was one of the best. That was at the University College School in Frognal, not far from here, where the Prime Minister's sons went. It was a day school and a public school as well.

"I used to go and watch Iain playing rugby but when I went to watch David Keith it was funny. He always seemed to be watching all the time, stage managing from the edge. He never seemed to be in the thick of things, but looked as if he was taking part just the same."

As we were talking we could hear the strains of Dad playing his priceless fiddle in another room. Just like he did when David was little. In those days he used to make David listen from one room while he tested a priceless fiddle and a cheap one next door and then ask him which was which. Young David used to have a great time with his Dad and I could see why. He has a very gentle voice and obviously an equally gentle nature. When he came back into the sitting-room, a large, high-ceilinged room with a huge bay window overlooking a small garden, he told me about his breakfast companion. A squirrel. "It's a little grey thing," he said. "I don't know where it lives but he has an uncanny instinct for knowing when it's breakfast time. I leave the window open and place a tin of nuts on this table," he pointed to a highly-polished table decorated with a vase of flowers just underneath the window "He's a nervous little fellow, very highly strung. If he hears a noise he's off...whoosh!" "He certainly knows when he's well off," said Mrs McCallum. "Those are expensive nuts. We used to have a Siamese cat who lived until it was thirteen or fourteen. You're not supposed to keep animals in this house but we had him written into the lease."

Mr McCallum then took up the story of David Keith (although I noticed David's Mum calls him that all the time his Dad calls him plain David). "David was born in a lovely big house in the suburbs of Glasgow," he said. "Then he's travelled all over the place wherever my work has taken us. He used to come with me to Shepperton, Elstree, and the BBC studios when I was doing background music for films and plays. Once when he was only ten or eleven, he sat in the same row as George Bernard Shaw listening to the music for Caesar and Cleopatra. They were the only two people in the whole place. It was quite an occasion for a little boy.

"And once when we were at Glyndebourne together I saw him painting ballet shoes. He knew they had to be painted so he just did it. He was painting very carefully - he always took a pride in doing things well. A famous director who was there at the time said to me, 'That boy will be a success one day' and how right he was.

"David also used to come to broadcasts of Music from the Movies which was a live radio programme with the stars acting excerpts from their latest films. He used to watch people like Stewart Granger and Phyllis Calvert doing short scenes at the Paris Cinema. He would sit in the front row of the stalls and get soaked in the atmosphere of it all. Those childhood experiences left a profound impression on him. He's not the sort of boy who would forget something like that. He kept it all in his mind.

"And he was always very quick at picking up things as a child. He has an instinctive flair for anything practical. For instance, he could mend anything. Once there was something wrong with Iain's electric razor and David said, 'Let me have a look at it' and he got it going again. If something went wrong with anything, the lamp for instance, he could mend it. He will get a fountain pen to flow again. David can become alive to things, he seems to see the object in a new light to anybody else."

It's no wonder David says he is still very close to his parents. They're a very young-at-heart couple with a terrific sense of humour coupled with a typically Scottish down-to-earth approach to life.

Mum is proud of her son's success but she's not over-awed by it and has until now resisted getting involved in the inevitable publicity spotlight that seeks out every famous son's mum.