Thanks Nico,
I have given it back to my dealer who has returned it to Rolex.
I was just wondering if anybody had a similar thing happen to them, is it an idiosyncratic quirk of the twinlock crown? My dealer is adamant that it is not normal and the crown should wind on easily and not require several attempts to get it on etc.
Never mind, I'm sure it will come back good as new. My Seamaster is keeping my wrist company at the moment!

Regards Joseph-John.