This issue appears to have resolved itself somehow during the night whilst the ATV2 was unplugged. Plugged it in this morning and it booted up just as it should. Baffling. Hope it doesn’t occur again!
EDIT: Looks like I spoke too soon. Just taken the box back upstairs and after plugging in I get the reboot loop again. I wonder if this is something to do with installing the boot into xbmc. I can’t uninstall it right now as I can’t SSH in. If I get back in I will uninstall it and see if the problem reoccurs.
EDIT2: I left it in reboot loop for around 5 mins while tunnelier was trying to SSH in, after a number of failed attempts it finally logged in at just the same time that my daughter squealed from upstairs “It’s on!”