When I got it home again and started looking around it properly, it became obvious that no threadlock etc had been used, so the swing arms were coming undone and the suspension going out of alignment.
Once I'd stripped down the suspension, it was obvious from the rub marks on the wheels just how far out of true it had become.
Here are the wheels, spacers and axles lined up. They're not much further apart when on the tank!
Having got the tank apart, I decided to bring it right up to date with the same spec suspension as the (then current) Mid Tiger.
Adrian.