Post back let me know what the lads at ocuk reckon
This is a bit late and a bit out of the blue, so sorry I didn't post back. Basically, after loads of messing about with replacements and loads of fiddling and tweaking, I just ended up accepting that the weird boot issue wasn't going to go away. Overclockers, although helpful, seemed to want to try and 'fix' it by getting me to do various tests/tweaks. I gave up in the end, as it just seemed that it was some sort of hardware/overclocking issue and I don't think it was ever really going to get resolved and all the messaging with overclockers and tweaking of the system meant I ended up trying to fix it more than I was actually getting to use the computer for what I wanted it for.
But anyway, the reason why I've resurfaced this thread is because I got some extra RAM for my system recently and, so far, I haven't had the weird/random boot issue since. Now though, I have started getting overheating (80°C+, then bluescreen/system restart when under load) so I just came back to see if anyone actually found any solutions or fixes for their issues, out of interest, or to see if anyone else has upgraded their system and ran into overheating issues?