Thanks for the advice.

We do normally keep biscuits down, I think I'm just going to make sure that there are more than enough down before we go to bed.

OH has suggested we keep them out of the bedroom for a week so they don't associated the bed with peeing. I've also got some Simple Solution for cat smells that I've sprayed on everything that came into contact with the pee...

Guess I'll just have to hope that they'll either stop or grow out of it, otherwise they'll have to stay out of the bedroom, which would be a shame as they've just started to sleep on our feet ...