Was the NS bottom ball joint. It was only slightly knocking, so it can't have been totally worn, but the effect it had on the steering was unreal! I've had cars before with ball joints needing replaced but I've never experienced that sort of effect.

Anyway, it's been replaced, cost £68 fitted, my steering wheel is now dead level and if I let go of the wheel, the car doesn't almost do a uturn. Only making this a thread as I've noticed quite a lot of 'steering pulling' ones. If anyone else has had this and it turned out to be something different causing it that would be interesting to know.