When you do contact them I'd also suggest Adam that you kick up a great big stink over it. Not only did it need the motherboard replaced - "and for the price I paid I'd expect it to be more reliable when you replaced it you made a pig's ear of it, causing me to have to investigate and diagnose your inept work, and putting me out by all the hassles of returning it to you again so you can fix your sloppy work."
You might as well try for some kind of apology/freebie/something from them. They haven't bothered to check their own work properly and it's rather poor of them.
Thanks everyone for helping me to ascertain that it was in fact a fault with dells work and not me missing something obvious. I will be ringing dell and kicking off big time, You don't normally expect a computer that cost £800 to have so many problems, being at Uni and having to send my computer back for 2 weeks was extremely inconvenient and now due to their poor work I have to wait another 2 weeks to get my laptop back.
Something else that points to a fan or the heatsink not being correctly installed is that the backlit keyboard no longer works and the volume function keys don't work either, my guess is that the person that fixed my computer was inexperinced or just rushed.
Is there a chance of Dell just sending me a new laptop? Not only because of the number of times ive had to send it back to dell but because of all the nagging issues with it that I was willing to put up with. The computer is well known to have wireless issues e.g. you cant sit further than 10m away from the router it also frequently overheats and while the wireless card is on sound is severly distorted.