OK, I've stopped sulking now
and ordered that replacement crank off ebay. Too far away to go and see, but I had a good chat with the seller beforehand and have to trust his promise of my money back if the part's no good. Fingers crossed it's a good un.
When I stripped the engine cases first time round, I think the gearbox was missing some spacers/shims on either end of the output shaft (the one the drive sprocket sits on) - there seemed to be a lot of end float - about 5mm I guessed at the time.
When I rebuilt the box, I put a 3mm shim on the output side which seemed to centralise the pinions and the gearbox shifted OK when the cases were bolted up. There is still just over 1.2 mm of end float. Is that normal or do I need to shim the 'other' end too?
I'll have the cases split again when the replacement crank arrives, so I'll do some photo's of what I've got so far.