Probably a bit too late to add anything now but I have two cats and whilst they had two litter trays one of them would insist on weeing every now and again behind the sofa. One of our cats is very domiant and when I was home ill one day I caught the dominent one guarding the litters trays like her life depended on it.

So to stop this we now have one litter tray upstairs in the back bedroom and one in the kitchen corner and now the dominant one struggles to guard both of them so she just doesnt bother and we have had no more accidents, my trick was to have them so far apart that there was no time for her to run from one to the other to guard! and that gave the other cat enough time to do what she needed.

cheers for the advice guys - all taken on board

Just thought id add this as things seem to have got a bit worse......

I found out who the culprit is the other day, it appears it was the older cat (stella) who is weeing in these particualr places (at the bottom of the stairs and by the front door). We rung up the vet and she suggested that it sounds like Wazza is being very dominant toward her (what with him running up to the litter tray while she is in there and hassling her).

The weeing is now happening a bit more frequently, we cleaned all the hallway with bio detergent and it appears to have had no effect. So now we are left with the ooption of gettin a feliway diffuser or vetting it for a possible Urinary tract infection.

???? What should we do next?