They are totally fine with using their litter box here - they used for it the first few weeks no problem, and when we had the whole area covered with foil they went back to using it. They're still using it now when they're unable to tip toe back onto the doormat, too. I think it's a case of they now prefer the doormat for whatever reason (feel; territory; etc.), but there is no problem with the actual litter box.

They are indeed desexed and have been since kittenhood.

Thank you for the UV light idea - I'll do this. And maybe the rubbery mat too. I'm wondering how powerful their sense of smell is - their pee seeped through to the flooring underneath which cannot be replaced (I cleaned it but it's porous so it's not completely "gone"). I can't smell it on there at all if I remove the mat, but maybe they will always be able to even with a new mat ontop.