Reporting in
I gave Santosh's suggestion a try but with no luck.
Now for the good news, I have managed to resolve the issue.
I basically started from scratch, detached the LUN, removed the mappings from the SAN, recreated a new mapping on the SAN with a new LUN number, re-scanned on all ESX hosts and then attempted the exact same process to add the RDM to both nodes.
Who knows but this time it worked first time.
Thanks very much for the help chaps.
Cheers
Simon