After the intervention of @Kain_W from the VM support team, and an exchange of several PMs with him, a visit by a technician was eventually arranged. The fix was a replacement for one of my 2 V6 boxes - once that was installed and initialised my second V6 box auto-upgraded straight away.
Patience is needed with the VM support system but their technicians who come to the premises to fix a problem are usually very good.