When Michael Portillo walks into the restaurant on the 28th floor of the Hilton hotel on London's Park Lane, Galvin at Windows - the first thing I notice is that he's lost weight.

The next thing you wonder is whether he looked better in his old guise. Portillo, 58, was never a porker but I'm not sure that skinny really looks good on him. Later, as he poses for our photographer, he explains the diet he's been on. Nothing too severe, he says. 'I have simply been eating no bread, no butter, no pasta ...' his voice rises in mock exasperation ' ...no potatoes, cheese, puddings, cream. I just eat protein basically. Do-it-yourself Atkins.'

He's been on the diet for nine months now; this is clearly a man of some discipline. Today he's kept to his eating plan more or less, refusing bread and opting for the salmon followed by lamb, washed down with a glass of champagne. 'I love food,' he says. 'Eating is one of my chief recreations.'

Dining at home in north London as a child was a curious mixture of Spanish and Scottish - porridge, paella, risotto - a combination explained by the fact that his mother, Cora, boasts a grand Scottish ancestry and his father, Luis, was a refugee from the Spanish Civil War.

Portillo remembers being sent to Spain unaccompanied as an eight-year-old child in 1961, to pick up the language - and, as it transpired, to turn up his nose at the chickpea dishes and peasant stews that were served up by his father's family.

'Spain was markedly poorer than Britain. There was no running water in the villages and people routinely travelled by horse and cart, even in Madrid. Of course I loved to go around on pony and trap, it was fantastic. But on the whole I didn't get on with the food. I used to exist on custard, largely.'

That year was an exciting one. Not only was he travelling abroad on his own, it was also the first time he achieved national prominence - seen by millions of viewers in a wonderfully stern TV commercial for Ribena.

Looking at the ad now - you can see it on Portillo's website - you might think you are watching an advert for dog food. The commercial is a succession of still pictures of a young boy supposedly making his way through life (in fact five or six similar-looking children) while a bossy voiceover credits the lad's vim and vigour to a regular diet of blackcurrant squash. As the camera closes in on young Michael's smile, it announces: 'Each day Ribena helps to build young limbs, straight and true, strong bones, good teeth.'

Good teeth? Portillo smiles at the 'slightly preposterous' claim. It wouldn't, I say, get past the advertising standards people today. 'No,' he agrees. But the ad 'was probably quite avant-garde at the time'.

The commercial came about because Portillo lived across the street (in Stanmore, northwest London) from an elocution teacher with showbusiness connections who regularly put children up for roles in TV and film. (Portillo tried for many other roles, including one in a film with Hayley Mills.)

The Ribena audition was 'disastrous', he recalls. 'We got caught in a rain storm and I arrived tense and looking like a drowned rat.' Yet, to his amazement, he got the job. On the day of the shoot, 'they simply said: "Play." There was a climbing frame and I just had to play and they snapped away.' He recalls his pride when the ad regularly played during the middle of Coronation Street, and his knowledge even as an eight-year-old boy that perhaps 15 million people had seen him.

And his fee? 'Five guineas - and two guineas travelling expenses.'

But, he says, you shouldn't hear this story of showbiz auditions and childhood stardom and run away with the idea that he was a very self-assured young lad. 'Showing off in public quite often goes hand in hand with a lack of confidence.'

But he had plenty of friends at school at Harrow County School for Boys (not to be confused with the rather grander Harrow public school up the road).

One of his pals was - and still is - Clive Anderson. 'I've known Clive all the way through because we were at primary school, secondary school and university together.'

Theirs was a warm friendship but a fiercely competitive one. They'd walk home together from school, trying to catch each other out - the future quizmaster gleefully celebrating when Portillo displayed his ignorance. ('We must have been about seven when he said to me: "Do you know what BBC stands for?"

And I said: "Yeah, British Broadcasting Company." Got it wrong. I mean that's incredible isn't it, at the age of seven we were debating things like that.')

The young Anderson came round for tea after school, for birthday picnics - and sometimes joined in some rather naughtier activities.

When he was a teenager, Portillo and a group of chums managed to persuade the school authorities to put him in charge of running a 'book room' - where textbooks were catalogued, covered and stored.

'This was a fantastic wheeze: there were two or three of us who took over this book room when we were about 13 and relinquished it when we were 18. So when everyone else was forced out to play or forced to go home, we had the book room to go to. This was our den.' A den of iniquity, it turns out. For Portillo's little gang used to load themselves up on the sickly alcoholic apple drink Pomagne in there. 'Oh yes, we used to get quite pissed on that.' In your lunch hour? 'No, after school. We weren't drunk for lessons.'

People might have expected the young Portillo to have been a little bit squarer than that, I say.

'Well I was very well behaved in the sense that I don't think I ever got a detention or anything.'

You should have done for that!

'Well it's funny, I think those days were more liberal. I think teachers were pretty aware that we were drinking in there. But we weren't smoking. Smoking was regarded as quite an important crime but I don't think drinking was particularly.'

So, I ask, was the respected television personality Mr Anderson sometimes involved?

Portillo smiles. 'He might well have been.'

My favourite table

Galvin at Windows, London Hilton, 22 Park Lane, London W1, 020 7208 4021, www.galvinatwindows.com