He's tugging the fur out from round the base of his tail. I've combed him and found no fleas and the skin doesn;t look irritated at all. He is a happy cat flap cat and ther is no change in his life or in his behaviour, so I don't think it's stress.
Fleas would be my first guess, even though you've combed him, they're easy to miss! Anal gland impaction could cause irritation but he'd be more likely to be licking at his bum rather than the tail base. The other thing that springs to mind is a cat bite, the tail base is one of the commonest areas to be affected and you can't always see the puncture wound until it starts to swell.
I'm afraid you'll probably have to wait till tomorrow to find out!
Oh and when you say well flea treated, the preparations and formulations you can buy vary massively in effectiveness (and safety but that's a whole other thread!) so if you're using a flea collar or supermarket useless crap products, fleas can still be an issue!