After an horrible journey to the Isle of Mann with the Zumo freezing many times, it behaved absolutely fine for days of local rides with only two notable differences:
- one of the motorcycle mount spring loaded gold plated contacts was stuck down and I freed that
- the plastic bit between the contacts broke away (one of the recommended fixes in this thread)
Could the issue be related to handling multiple short power interruptions?
Returning the unit for exchange as previously arranged anyway, but I guess it's not much use as pointed out by others!