Right! About time that I gave you all an update of what happened!
After talking to an Apple technician, we were pretty sure that it was the PSU that blew.
Unfortunately, Apple said there was nothing they could do, and because I hadn't bought a 3 year Apple Care protection plan at the time of purchase it looked like I was going to have to pay for any repair, costing me between £250 - £280 (£150 for the part and roughly £100 for the repair itself).
I was about to bite the bullet, however, just before I left home to take it to the repair shop I thought I'd just check the ins-and-outs of an Apple warranty etc. online.
It's a good job I did, because I found out that something that may be useful to a few of you out there!
If you purchase a computer through the Apple education store (which I did), your machine comes with a "special three year parts and labour coverage, with additional first-year warranty and support benefits, as standard".
Awesome.
So, I rang Apple support again just to check this out. They confirmed that this was indeed the case (although they had neglected to tell me this originally), and all I had to do was take it to a repair centre and they would take care of the rest!
Woohoo! A happy ending!