hmmm - I might have solved this one. I've put an exposed piece of film onto a spool, and what do you know the bottom edge of the image is clear of the plastic spool, but the lower edge is partly covered by the plastic on the spool. This is probably causing the problem?
Bit of a shame, as although the Rollei is giving me 57mm on the rebate, I'll have to frame a bit looser in some circumstances.