Went for follow up appointment at the vets today. He wasnt happy that Fred displayed all symptioms consistent with anal gland problems and wouldnt consider removing them until all other avenues had been explored. He asked my permission to shave freds back end, of course i said yes, low and behold - the reason fred chews is nothing to do with his bum, but that he has an isolated skin infection on the base of his tail and towards his bottom :-) Have got anti inflammatory & antibiotic ointment to put on twice a day for 2 weeks to see if it improves. Very impressed with my vet, Freds not though.... thanks to the vets less than impressive clipper skills he looks like he's had a haircut from a blind person :-)