Thanks for the quick reply. I think I've made some progress.
As you suggested, I checked for hard drive errors and
Windows System File Integrity violations, both came up with no errors.
So I removed my GTX480 graphics card and replaced it with an old geforce 9 series card then installed the new drivers. This seems to have fixed my BSODs and I was able to play Red Orchestra 2 for over two and a half hours without any problems when previously I could only play it for no more than 20 minutes before it blue-screened.
So fingers crossed it was a faulty grahics card.
Thanks for your help.