Do the spot-ons kill the same worms? I know advocate kills one kind buti didn't know there were spot-ons for the others (I would much rather not use tablets - I've got the next dose already from previous vet visit so I'll give it a go but if I fail hopelessly I'll try an alternative )
Cats are always likely to pick up fleas. Even if they never go out and never mix with other cats. You've been lucky that they haven't so far but you can't guarantee it. It would probably be an idea to treat them before the cattery, or you could just keep an eye on them (with a flea comb) when they're back and treat if needed. If you're not going to treat regularly then you will need to do that every week anyway to make sure they are still flea-free.
I used to treat them with Advocate but they have been flea-free for months. They do go outside, but not far, & there don't seem to be any other cats around (apart from one they hissed & yowled at from the top of our wall - just the once )
Anyway they're boarding for 10 nights later this month. Should I get some Advocate in just in case?