Harmy said:

Much appreciated guys - it seems the returning of the broken motherboard won't work out (they found that some of the pins on the CPU bed were slightly bent and tey said I must have done it and gave it as a reason to not refund me for the mobo, even though I'm convinced it can't possibly be what was causing the problems, which didn't appear until I put in the additional RAM), so this will help a little to cover the cost of the new one.

I had exactly the same problem a few years ago Never went back to that store.