Well apparently the position I wrote to, which I'm calling BIOS1, the one closest to the DVI ports, is supposed to be unwritable and I've somehow managed to write to it.
In the end I just flashed the Sapphire BIOS into BIOS2 and it's working as a 6970 now!
The 6970 BIOS in BIOS1 sort of works, it boots the system but can't drive my second monitor. Now that I understand that BIOS1 is not supposed to be writable I don't feel so nervous about flashing BIOS2.
Would love to know how I managed to write to BIOS1 though, maybe the card was shipped with the switch in a half way position, enough to select the address line for BIOS1 but not to enable the write protect.
To find this out I'd probably have to take the card apart, and frankly I cannot be bothered.