i changed the belt and also the crankshaft pulley which was starting to break up, i checked the rollers on the belt tensioner top and bottom and both seemed ok.
i've noticed the noise has started to come back and this time the belt has jumped partially off the bottom pulley and it's flapping around on tick over, it's fine once you put a few revs on the car.
has any one replaced one of these tensioners , not sure which part it is i need but i guess it's the bit with spring in it