Now 72, Blake looks much younger. His drawing style changed, he says, when he was 23. "That was when I discovered that you could be relaxed and alert at the same time; you could stop worrying - concentrate rather than worry. That was the big change.
"It came about initially because I used to do roughs to send in, and sometimes the roughs came out better than the drawings, because they had more life in them. So then I started to try to build some of that spontaneity in."
Many children's authors, Blake says, go back to their own childhoods for their stories. "They somehow have access to their childhood, whether it's Michael Rosen or AA Milne, they seem to get back to it. I never feel I'm doing that, in my books - what I always feel I'm doing is imitating children now. You see, I don't draw from life at all, but I do look out of my window a lot."
So, are his drawings reflecting changing childhood over the years?
"One has some sense that they've changed, they wear different trousers ... " he says, musingly.