VICTORY IS MINE!
@dc42 I had come to the conclusion it was a comms issue causing the reset as I had traced the fault to only occurring when the Ethernet port was connected to the network.
I've given the board a static IP address and that seems to have cured the problem. I'm afraid I was unable to look at the "last reset reason", the board just didn't want to report back.
I will give the new firmware a go at some point, first of all I want to finish putting the printer together and try to get it to print something!