***This has been solved, post for others who may experience it.***
Late yesterday evening I returned hone and had a desperate message from the secretary of the local cancer support charity for whom I provide email facilities and web support. The email server seemed to be refusing emails in or out.
I investigated and saw that the VPS had run out of storage space. My first act was to try to uninstall some programs thet have been there since Adam was a boy. But apt would not play ball. No storage space meant it could not do it. So I firstly contacted Andy to request some more storage space, but he would not see it, of course, till the morning.
Next I trawled through manually hunting larger text or data files I could happily lose. Interestingly fail2ban logs and the latest archive of them were very large, they were sacrificed. The emailbox that is used to collect confirmation of DKIM was busting at the seams, they all went. I will change the config so that I wont get confirmations anymore. 6 months of it working seems to be sufficient. Various other gains were made which should have provided enough space but nothing was happening and I was falling asleep at the keyboard so gave up until bright and early today.
This morning Andy had already activated extra storage for me (as well as giving loads of tips), but it needed activation at my end, which required installing parted which I could not do as apt could not access any storage. So I decided to try rebooting. (always the first thing recommended) It went down OK but would not reboot. Dead as a dodo. Undaunted, I fired up a Xen terminal and rebooted from there. It worked a dream. and the recovered space was there, so I could now use apt to remove old kernel modules and do an autoremove, install parted, get the new atorage recognised and start on the slow process of tidying up the disc space and so on. I fully intend to ask Andy for the Icinga monitoring service so I don't get this again.
What a fun day, but a day of learning