Whilst tinkering with lvm (which now works great - thanks!), I made a typo which caused the domain to stop booting. I think it was just that I got the config file wrong in terms of which device mapped to which.
Anyway, I had specified 'restart=True' and so the domain was starting to boot, failing, then restarting again. This was the expected behaviour, but it was a pain to kill. Had I thought of it I would have just killed xend but I was doing an 'xm list' to find the domain number, then 'xm destroy', but by then it had moved on. doh!
How easy would it be to put in some logic to detect a domain restarting rapidly in quick succession and either stop restarting it, or introduce an increasing delay between restarts?