Solution: Don't take them to a cattery again.
OK, that's not all too useful. Sorry, it's just my twisted sense of humour kicking in there.
We use Frontline {I think, ask MelodySK, my mum} to make the fleas sterile.. it tends to work. You just use it over and over, once every so often {it says how often on the packet} and they should, eventually, go away.
We used to have a huge problem with fleas, because we stopped using the stuff. We started about six months back now, and voila, rid of fleas

Hope it helps ^^
darshana :P