If I lived anywhere near Keighley I would help you to do the leg work. Easy to say I know but those who know me, know that i am sincere and will do almost anything for an animal, especially a cat.
Are there any clues from the collar or where it was found? Is it worn or chewed or mud stained? That may indicate that it has been there for some time. Where was it found? on the ground ? On a branch or twig? They may seem strange questions but I believe in leaving no stone unturned in looking for a cat and plan finding them like a military exercise.
Whilst I haven't read all this thread, I assume that you have re-checked with all the rescues and vets in say a 5-10 mile radius. Was he chipped? If possible physically visit the rescues in particular and take a series of photos to show markings from the front, side, lying down, etc.
I really hope that Smudge is found, Whoever passed you the clollar sounds like the sort of person who restores your faith in human nature,