His behaviour started to deteriorate at around 18 months to two years. He'd gone to the same nursery that Natasha, our eldest, went to. There was one teacher who was very fond of him but one day he hit her with a Dinky car and scratched her face. We were called in, and they said, "Look, it is a one-off. But he's having some behaviour problems. He never wants to do what anybody else wants to do; if you try to take him away from anything he's trying to do, he reacts very badly. Maybe you ought to get him checked out."
His behaviour at home was extraordinary. He was the perfect baby for sitting down with a toy; he wouldn't need any interaction at all. Then he started to do lining up. He would make abstract patterns on the floor - concise, geometric patterns that almost seemed to have a cryptic meaning. The lines were so straight you could put a ruler down them.
By 18 months, he knew and could say his alphabet, backwards and forwards. He could read certain words - but his only speech was "Mum", "Dad" and "No". "No" was a big word. Yes didn't enter into it.
At two, two-and-a-half, his speech was very slow. I think in retrospect, I was in deep denial that there was a problem. Olivia by then was pregnant with our third child, Claudia. But I think she knew. He was going through a crying period. All children cry. But this screaming never seemed to stop. I was off working - filming and in the theatre at the same time. But it came to the point where Liv said, "we've got to do something."
So we went to see our GP. Just before, the strangest thing happened. We were in our sitting room and there was a book on the shelf, John Cleese's book, Families and How to survive them. I picked it off the shelf, and it fell open, literally, at a page about autism. It described all these features, and I thought, "that's Bassie"; it was describing our son.
I didn't say anything to Olivia, but we went down to our GP and I asked, "Is there any chance he could be autistic?" She said, "there's absolutely no way this child is autistic; he's too sociable. But I do think you ought to go and have a developmental check."
So we went along to our Richmond borough paediatrician who gave him a quick check and said, "He's obviously a bit behind. He's about six months slower in his speech. But he's a very bright boy. His IQ is off the scale." And I thought, "That's my boy."
I remember us thinking "well, so he's a bit slow; but he'll catch up." But something was nagging away at me, and a friend suggested I get an appointment with Gillian Baird, the developmental paediatrician at Guy's hospital.
It was a Saturday morning when we arrived at the hospital in Leatherhead; we were due to go on to a wedding straight after, to a friend in Oxford. I can remember the hospital being all decked out for Christmas - all these silver plastic Christmas trees twinkling in pots.
We were called in to see Gillian and a speech therapist who immediately started playing with Sebastian whilst Gillian took a family history. Out of the corner of my eye and ear, I was listening to what this therapist was doing with Sebastian. He was holding his own, really - his language wasn't great but he was doing everything asked of him. Then Gillian did some psychological tests - and he just ripped through them. She even got to a point where she said, "I don't have any more; he's done them all. You're such a bright boy." And I was literally screaming inside, "Go on, go for it, get the bastards, do it, do it."
Then she said, "OK." We were sitting next to each other. She said, "What do you think the problem is?" And I said, "That's why we're here." She smiled and said, "Yes, but what do you think the real problem is?" And I said, "I thought he might be a genius." She said, "Well, he's certainly not stupid. But what do you think the real problem is, Daniel?"
Olivia replied and said, "We thought he might be autistic." Gillian said, "Well, I've got some devastating news for you. He is." I said, "A genius?" And she said, "No, autistic."
I remember glancing at Liv and everything going into slow motion. I asked, "Will he ever be normal?" And she said, "What's normal?" And I thought, "normal" is doing all those male things you're not supposed to talk about: kicking balls about, shagging girls.
We stumbled out of that hospital and looked at each other - it was like being in a film. We were waiting for someone to say, "cut"; we laughed - and in the same breath, crumbled, inconsolably. And yet we had Natasha and the baby, and Sebastian didn't look any different. We went off to this wedding and people said, "How did it go?" "Not too good really; he's got autism."
In retrospect - and I've heard it from a lot of people - it's as if the child you thought you had dies at the point of the diagnosis. A new child is born who has exactly the same love input, the same amount - if not more - caring. But it's a different child to the one you thought you were going to have.
We started researching autism. It's a very difficult subject to get your head around because it's so alien to what I and Olivia do: it's all about not being able to communicate.
Soon after, we bumped into one of the girls at the nursery where this had all started and she told us about a new treatment called Lovass - pioneered by a psychologist at UCLA called Ivor Lovass, a practical, one-to-one therapy that takes over your lives.
To cut a long story short, we embarked on this 30-40 hours a week, intensive, education, therapy and speech language programme where everything is broken down into tiny units and is rewarded. Sebastian completed the programme six months ahead of time: it should take two years; he took 18 months. The first day he worked with a consultant was the first day in his little life that he said "yes". It was a revelation - and it was done with a tiny piece of chocolate.
He now goes to a normal school. The treatment was expensive but more councils are now willing to pay. We set up a charity called Peach (Parents for Early Intervention in Autistic Children) that gives information on Lovass and is going to set up a centre, in London to begin with, to promote the therapy that's worked so well for my boyn
Daniel Hill's 'Cracked', about a British psychiatric unit in the Gulf War but informed by his own experiences of what he calls 'the autistic war' previews at Hampstead Theatre from 10 April and opens on 15 April (0171-722 9301)
Peach, c/o PO Box 10836, London SW13 9ZNReuse content