I finally managed to restore my system to something usable but it involved reformatting the hard-drive and re-installing Ubuntu. Still it gave me the opportunity to re-partition the drive more sensibly.
Originally I had one primary partition and one extended partition (with a couple inside) and the remainder, about 800Gs for datastore. That’s not too useful if you want to install several O/Ss and boot between them (using Grub). At least Ubuntu, OpenSolaris, Centos, Suse. vSphere boots from a USB device but it didn’t like having it’s datastore pulled out from under it. It does fully boot eventually but there’s a long pause at the end. I will need to look at the logs. I wonder why it wouldn’t let me delete the datastore before, even when I had deleted all the VMs and isos? Still like to get to the bottom of that.
I downloaded Centos to a live USB but it doesn’t have the luxury of installing. In fact it looks a bit fiddly, involving booting over the network and playing with Grub etc. (no DVD drive). Not something I bargained for but all good practice I guess.