Identify this book for me?

[onetiredmummy]

I read it when I was little & it was illustrated by Quentin Blake although I don't think its a Dahl.

It was about a boy who was epileptic & discovered he enjoyed swimming. They class once swam in a river.

I know I know its a crap recollection, sorry. Any ideas welcome :)

Here?

You are amazing, how did you find that?! Thank you! xxx

I'd like to tell you that as an ex-academic librarian, I have super book finding skills.

But I googled.