There is nothing in any logs at all, on either the Dom_0 or Dom_U at the time of the crash, the journal on the Dom_U stops updating at the instant of the freeze and only resumes after it has been destroyed and re-started.
We are in the process of moving sites to HVM, that is an ongoing part of our plan, 1 down...
We've not tried running a screen session, but that is a very good idea, and we'll do that - thanks! I don't believe we've tried running in debug mode either, so will pursue that too.
Thanks for the advice, I appreciate it - I know it isn't anything even remotely related to your work, but as an XCP-ng user personally, I wasn't sure where else to try.