Have tried that numerous times.....
In the end I rebooted the vCenter Server..... and that cleared up some stuff and allowed VDP to generate a different error which explained what was causing the problem.
It seems one of my colleagues had 'unmounted' a datastore from one of the hosts and VDP was kicking up an error similar to the one in this KB: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2037003
Jumped onto my storage array and unpresented the LUN from the host and VDP kicked into life......
Man.... this product really needs a revamp!