Problem resolved.
The clue was obviously in the engine.log (FSCK error) but I couldn't see
what was needing an fsck or why this would prevent the storage of every
vm from being mounted.
I still dont understand the whole of the ovirt storage mechanism.
More information was in the node vdsm.log - It pointed here.
/dev/mapper/9ed5a2fe--ae4c--433f--a15f--2c710ed5156f-master
One fsck later problem resolved.
Thanks
Andy