I couldn't remember seeing my superhub reboot for a while so decided to check and............
What a turn up. First time I saw the measurement period rise above a few hours.
So it's early days to say my superhub rebooting problem is fixed but as absolutely nothing has changed with my kit, indications are the rebooting issue was with VM's network and not the superhub.
If this is the case it shows that a previously solid docsis1 network experience should in no way be taken as an indication as to the status of the docsis3 network.
Anyway, in all fairness, it would appear that in this case, at least, random rebooting was not a superhub issue.