I know the cat you mean, we live in Grierson too. He caused a fight between our two cats in June last year and we had to rehome one of ours as they never forgave one another and kept fighting, despite all our efforts including a consultation with a cat behaviourist. If he really is a stray and we didn't think he was because he looks so well, clearly from robbing yours and other poor cats food, you could contact the cats protection league and ask them to put down humane traps to catch him.