Hi there, thanks for the replies
I'm not sure how exactly they are getting dirty bums, as their poo is solid. I think its a matter of technique, and being as they are both 9 weeks old on Sunday, they haven't quite got the "mechanics" of it all down yet. They are both short hair kittens, and I think that its just that they haven't cracked the whole angle tail, squeeze, shut and then clean.
I'm just giving thema wee helping hand until it clicks.