I have recently done a complete system rebuild, with everything new (cpu, memory, m/b, even the psu) except the gfx card. So i have tried again, and it's the same crash, running the latest version of Arma, and the error has now occurred in World Of Tanks, so it's not a problem with the software.
I have spent a long time on this, and I now believe it to be a hardware fault on a memory module. The reason it's taken so long to pin down is that arma is a heavy user of the memory on the card due to the details in high quality modes, and most other games were not at the time, but are starting to catch up now, so it's only hapenning in other games now.
The card is 16 months old, but the company I bought it from have confirmed they will still RMA it as it's a hardware fault, so assuming that goes ok, I think we ca close this issue.
Thanks for your help with trying to get this sorted out, and hopefully, I won't be raising another one in a few weeks when I have a new card!