Mikey
Thanks Bear, Yes I set up a GUID partition.
Update: I set the WD 750GB drive in a USB dock and connected to another MB, I then used CCC to copy its HD to the WD. That worked OK, in fact I found that I could now boot from the WD via the dock and that Disk Utility now correctly recognised the WD as a 750GB disk. However, when I reinstalled the WD internally in the MB it refused to boot. I tried to boot from the OS X install DVD but that wouldn't work either.
Good news - The WD 750GB HD is now bootable and is working well. I connected the 2 MB's via Firewire and used Disk Utility to reformat the WD, (one odd thing the partition table was showing as an Apple table?) I reset it to GUID and everything worked from there.