OK, I ended up messing about with too much, so I wiped and started again today.
The problem I have now, with trying to get a network boot, is in the webadmin page, I have the external NIC as eth0 and the internal as eth0:1.
When I try a /etc/init.d/networking restart, the eth0:1 device doesnt get recognised.
Also, in the web admin networking page, eth0 and eth0:1 both have the same mac address.
I am guessing there's an mce file I need to amend somewhere?
Currently, I recieve a DHGCP address on the diskless MD, but then follows a few PXE errors (E11 ARP timeout, E38 TFTP can't open a connection).