Solved this. It was a disk space issue. I thouhgt I had plenty of disk space but not so.
If doing an update on a digital Ocean Ubuntu Droplet it is an idea to first run
df -h
on the Droplet and checking capacity.. I don't know exactly how much is needed, but certainly elss than 5GB free you will run into difficulties, and may require up to 10GB free.
I also benefitted from doing a docker system prune as there was quite a lot of rubbish in the volume, but be very careful, because depending on your architecture you may be deleting something useful.