Okies, we progress. Back from an hour and a half troubleshooting session with "Firedog" self-proclaimed gurus at the high street mega-chain store.
Gurus highly complementary on all aspects, equipment, case, build technique - everything. All looked 100% (but didn't work, lol).
The motherboard obviously has two PCIExpress video slots. One blue, one black. One card in each, bridge across for SLI - nuth'n.
EITHER card in blue slot only brought us to full boot up and bios showing on monitor - so both video cards are good. Yay!! Any attempt to do anything with the black socket (either just one card in there on it's own (nothing in the blue one) or card sat there but not bridged) causes complete failure of the boot.
The guru's instantly go for "faulty slot on the motherboard, send it back for replacement".
Machine is now at home, running on one card, and loading Vista files from the CD.
Graphics cards, by the way are XFX, nVidia GeForce 8600GT. 512mb, DDR3, 128 bit, 540 clock, PCI Express, SLI Ready. (No idea what any of that means, lol).
There is no speaker in this case - so "No encouraging beeps" ever heard from it.