Thanks André - I agree, the mac address conflict is causing the problem! I really don't know how on earth I have managed to do that!!?
Anyway - I guess I need to chnage the mac on one of my ESXi hosts. But the problem is I am afraid I don't know how!? 
My two host servers are physical machines (they're Fujitsu servers) and only have a single NIC in each at the moment. I just can't figure out how to either change the mac address, or remove the nic and re-add it so it gets a new address?
Any advice on changing the mac address would be much appreciated!
Thanks in advance 