Many thanks for the advice. I have tried both of the cards in the primary/master position now, taking all the required special care to avoid damage to the chips. While I was there, I also carefully replaced the thermal compound between the chip and the heat spreaders etc.
The result, however, is the same in every instance - when both cards are connected, they both show up in Device Manager, the (top) one appears normal while the second appers with a little yellow triangle, indicating that there's some sort of problem. With either card in the master posiion (with second card removed), it shows up in Device Msnager with the same yellow triangle. With either card in the slave position, there is no display on booting (as expected).
In each of the scenrios above I removed any trace of the nvidia driver using the Display Driver Uninstall Utility, before restarting and reinstalling the driver from scratch. This seems to make no difference, and neither does trying to update the driver manually, or rebooting after the nvidia driver is installed. In all cases the card(s) fail to be recognised by the nvidia driver, and the card seems to fall back to using the Microsoft Basic Dispay driver without exception.
I've been using one of Prema's bios mods for over a year (kudos to Prema!) and it's been working excellently all along. Could it be possible than I will need to flash a different version, or possibly stock?
Cheers for helping with this.
Another thing I didn't mention but that may be of major significance: I have just recently reinstalled Win 8.1 pro on a single ssd, this time using UEFI mode (previous install was mbr on a twin-ssd RAID0 set). Might this be where the root of the problem lies. It might be great news, as this might actually mean that both cards could actually still be working? Please correct me if I'm wrong, your thoughs are much appreciated