Its really odd!! Thinking back, I had loads of problems getting ESX to install on one of the servers because it had on-board RAID and 4 disks - and the ESX install had a problem seeing a volume to install to. Anyway - in the end I installed to a USB drive, and booted from that. But I guess I swapped the USB drive to the other server!
Anyway - these servers only have one on board nic - so no chance of removing I guess 
I shall do a re-install now. I guess that'll fix it! I will report back.....
Thanks very much for the advice!