Thanks for the suggestion. I can confirm that if I try to set the EVC mode to anything other than 'Gen 3 without 3DNow!', I get the following error;
The host's CPU hardware does not support the cluster's current Enhanced vMotion Compatibility mode. The host CPU lacks features required by that mode.
If I try to set the EVC mode to 'Gen 3 without 3DNow!', I actually get a different error;
The host cannot be admitted to the cluster's current Enhanced vMotion Compatibility mode. Powered-on or suspended virtual machines on the host may be using CPU features hidden by that mode.
The only VM I've got running on the host at the moment is the vCenter Appliance, and I can't shut that down, or I won't be able to change the EVC mode! Only way I can see is if I migrate the appliance off the host, then try setting the EVC mode again when the appliance is running somewhere else. Will that work?