If you require a new main belt, then Colin Braddock tape recorder centre mentioned earlier in the thread sells the complete set of belts for the 4307/4308 for £18.50 and will post them out to you within 24 hours. A bit dear i know but if you only want the main one, i am sure it should be a fare amount less. He is based in Blackpool.
His number is 01253 345049.