So, I finally got round to booting the other three new servers we'd recently purchased (these are nice simple dual core, dual CPU boxes (Dual-Core AMD Opteron(tm) Processor 2216) on Tyan Thunder n3600M (S2932) Motherboards)
I'd always been exceptionally suspicious of the PXE MAC address thet the 1st one offered (60:50:40:30:20:10) rather than the imprinted one on the rj45 socket starting with 00:E0:81 but the fact that three out of four machines all had the same mac address meant a phone call to the vendor was called for.
The one that works has the 2.01 BIOS, the others were already shipped with 2.02 and downgrading didn't cure the amnesia. I don't see a utilty to reburn the mac address ont the machine, so have left that in the capable hands of the vendor for now. Googling seems to suggest that nvidia cards are somewhat prone to wierdisms like this. Bother.