Ian,
I'm having the same problem, I started changing brace height around. I then asked for advice and when someone figured out what I was talking about. Almost getting slapped on the bottom of my thumb, I received the following advice. "I have found that string slap at the base of the thumb/wrist is either too low brace height for the bow and/or a too stretchy string.
A more none-stretch string type could solve the problem all by itself.
String slap higher up the arm can be about form and whether your arm is into the string or away from it but that low down towards your hand means the string is travelling forward to slap you almost on the hand.
Mark in England "
This makes sense to me, because I had not changed brace height until this started. I also had not shot this bow in awhile, it was my first selfbow and first string I had made. The string and bow both have 700+ arrows through them. I'm going to make a new string tomorrow and try it. I'm betting this is going to fix my problem.