Thanks Ian, that's reassuring.
I didn't fit the original (was a new bike) but this one I put plenty of grease on all of the indicated points, round the seals etc. As it's my new posh bike I even shelled out for some fancy expensive grease that came in a tube (a tube!! like toothpaste! not a tub or a tin ... what is the world coming to?).
Maybe I was just unlucky and got a lemon last time. I'll try to take the old one to bits some time to see why it siezed. It's really stuck fast.
