Mrrzzy, I wouldn't go much by the lack of a collar but perhaps that is another of those us vs uk differences with keeping cats. None of our cats have ever worn collars (except sometimes flea collars) and I rarely see a cat with a collar.

We try to go by the circumstaces. In your case, it seems almost certain that the cat had a good owner at least until recently. I would try to reunite cat with owner.

If you do what I think you should do and you found yourself having to give the cat back, it's not the end of the world. I don't know/can't remember what I've been told about the situation in the us but I feel sure there must be animal shelters and cats crying out for good homes.