This is worse than I thought guys... had an expert come round this morning who brought another PSU to test out, but that didn't work, so we can rule out the PSU being broken. We unplugged each component separately to see if it would then work... still no luck. So I guess I could narrow it down to the motherboard or processor being knackered, but can't be sure since we couldn't easily test those. What is weird though is when I try to turn it on having switched the power supply on and off, the fan on the CPU does spin for a second then stops... any more suggestions on what it could be?

I might have to end up trying to get my money back on the warranty for the motherboard, keeping the graphics card, hard-drive, etc. and buying some bundle or backbone with a motherboard and processor already in place.