I've considered tagging him to see if anyone responds, but have been a bit hesitant in case, for any reason, the owner sees it and "takes it the wrong way", and this ends up compromising the situation...paranoid, I know, but that's what you get for caring!
He's very friendly, but is getting a bit more cautious (not aggressive) now his sight is failing. He's not neutered, which is why there's so much competition when he's in the house.
Unfortunately, I haven't seen him now for a few days, but if I do I'll try and capture him, keep him for a couple of days and post a 'found' notice locally, as you say...hadn't thought of that - thanks, sheila.