Thanks Jon, that's a really good idea!
Keep them coming people, I need lots more!
Stephan: suggestions 1 & 3 are interesting - although they would both need rewrites of the program, of course. For no.1, are you thinking of different shaped grid blocks, or square blocks but in a different pattern? (if the latter, I guess it's just a matter of setting some squares to blanks before starting.)
No.3 would be nice, but of course having the same pattern each time means that you could solve it slowly and then restart to get a good time. Plus I'd have to make lots of picture-puzzles!
As for no.2, I'll try to keep the <10sec puzzles to a minimum but I'll put them in occasionally for variety! In any case, I can't always tell how low the times will go until you experts play it!