OK, first the bad news. It looks as though there is no way around this. Basically the system thinks Exchange is installed, but it's not nor is there a version number so you can't uninstall it.
However after removing thee V8 registry entries then dismantling the cluster and removing the Node from the domain, I was then able to ADSIEdit out the entries for this server under the Exchange configuration. This allowed the Other Exchange servers to open the EMC without issue.
Now I just have to re-install the OS and Exchange. This time I think I'll leave off W2K8 SP2!!
"What happens if I untick this?"