i think i know whats caused the problem now. the old bolts and drums have some corrosion on them for some reason. probably my fault getting the bolts dirty when doing coilovers, rear axle bushes, brakes and a few other bits all at the same time at the start of summer.
new drum, bearing and bolts on one side and all seems tickedy-boo. just waiting on another drum to be delivered and ill have a car again