Just had a reply from the finder:
"we have a vets appointment for the cat tonight as he is painfully thin and his teeth aren't in a particularily good condition. I have registered now on all the below sites and a few additional so now its a patience game, have emailed all the vets and and shelters in the area as well, cat is currently in our spare room, complete with climbing tree, fluffy cushions running water and food (small quantities as the first day when he snuck in and finished a whole sachet of food and proceeded to lose it all outside a while later) poor thing."
Sounds like he is in good hands.