Thanks for the reply Michael
We have found out that the problem is actually caused by the January update (KB4480963 Monthly Rollup) This affects AMD Bulldozer Family 15h, AMD Jaguar Family 16h, and AMD Puma Family 16h second generation microarchitecture one of which our hosts use,
and is solved, (we must verify) with KB4490512.
However KB4490512 requires a reboot;
We are unable to live migrate between hosts using SCVMM or Failover Cluster Manager although we can cold migrate.
We have 400 plus live VMs over 40 plus hosts and could really do with a solution that would not involve us powering off our VMs
Regards
Andrew