Thank you!
They have already learnt the 'shaking motion' (via deodorant) and scarper when anything like that is shaken. The problem is, I'm not around when they do the peeing! I will def give lemons a go, but have already tried orange peel.

To save another answer I've already tried tin foil. It only works on 2 out of 3 cats. No.3 is quite happy to piss on tin foil despite a) the noise. b) the puddle around his paws. He just shakes them off against the wall leaving spray marks.
(Big Smurf Brush me daddy-O, Mon 27 Apr 2009, 22:44, Reply)