More Clustering
Got clustering sorted. It's NLB only more so...

A VMware Clustering Recipe

One thing is that a SCSI driver is needed:
Using the VMware SCSI Disk Driver for Windows Guest Operating Systems

Also got the secondary DNS working. I'd missed something basic - I hadn't allowed zone transfers!
Also fixed CLIENT2 with a 172.16.x.x address to check out if a user can log onto the domain while SERVER3 (FSMO) is shut down. Worked fine - strange as I don't remember making SERVER5 a Global Catalog...