If you already have the answers right, the 'check' button appears not to work, but actually it highlights only those cells which are wrong, so if you've got it right it won't highlight anything.
Ben: Maybe when you get everything right it could give you a big green tick or something?
I've changed it slightly so that when you click check it highlights correct squares in green. I might add a tick later, though that probably won't be until Monday.
Ben: Maybe when you get everything right it could give you a big green tick or something?