Just tested the sticks of RAM on their own, one got errors straight away and one is fine after 4 passes of Memtest86. I don't have time to do more passes now but considering the other one was getting errors within 10 seconds I think that's okay for now.
I'm now running off just the stable 4GB stick, and I've taken the other one out. I activated driver
verifier and tested things that were causing freezes before (USB bluetooth adapter, zoomable images in web browser) and neither caused a freeze.
Don't want to jinx it but it seems that faulty stick of RAM may have been causing my problems, so I will get it replaced! I will wait and see if I have any more BSODs as it's early days yet, and keep you updated.