Well perseverance paid off !
I first unplugged all the connectors and memory fitted in my micro server and reconnected it all just in case I had a bad connection. I formatted the USB and reinstalled the Nethserver iso after downloading it again.
I pulled the 4 drives I had in the front drive bay of the micro server and disconnected and reconnected the 250GB SATA laptop drive I have in the top of the server (where the optical drive would be if I had one) and then put the USB in the USB connector on the motherboard.
Leaving the 4 large drives out, I reconnected the network and powered up.
I chose the CentOS option from the splash screen, chose the hard drive option (sda) and this time it all worked and after about 15 minutes had a base CentOS server
After editing the CentOS config files to get the network connection working the nethserver bit all just worked as described.
Thanks for all your input guys, I have a new native Nethserver running on my HP N40L