Yes, I am loosening the stem pinch bolts first.

Cartridge bearings, yes, and they seem to rotate fine. As far as I can tell, everything seems in good condition, though the upper bearing was a little corroded when I uncovered it. Not badly - just looked discoloured probably from the ingress of water. I think the upper race was in a similar state as well. I wouldn't have thought this would have caused the problems I'm having but... I wouldn't be asking if I knew the answer!

Thanks for the prompt reply.