I think the issue is being caused by a missing network.
One of the upgrades of my test oVirt cluster went sideways and I endedup reinstalling from fresh and importing the storage domains from the preivous cluster.
I haven't created all of the networks that were in the previous ovirt install as they're not really needed at the moment.
The vm's that are throwing the html null error when trying to view snapshots have a secondary nic that isn't assigned to any network.
Regards,
Brett