Too late, it's back together! Haven't got a flywheel holder, but I put a metal bar through the back sprocket and locked it against the swingarm... I did it up as tight as I could get it, and I used a spring washer in there as well, so hopefully that will do the trick. The spring washer is a fairly thick, heavy duty type and brings the nut all the way out to the end of the thread, but it's on good and tight, and there's enough clearance for the clutch to work.
If it happens again at least I'll know that it won't do any damage, and I'll just have to do a better job next time...
Has anyone else tried a spring washer? The manual showed some sort of conical washer, but the exploded diagram at the start of the clutch section didn't show one and my bike certainly didn't have one.
fingers crossed...
Andrew