Ah - I've found the problem.
A screw at the back, near the power supply outlet and fan:
Great.
The cover now slides out incredibly easy and I rotated it round to place on top of the case while I work. No difficulties here. Had good wiring slack to play with.
I've now installed my fan. The rest was easy.
I used the SilenX 14 dBA - IXTREMA 40 6cfm - 20mm with the silicon mounts.
Simply unscrew the fan. I used some pliers and a screw driver. Place the silicon plugs into the hole - pointy bit going into the case and then place the fan.
I had to make sure that I have the air flow going the right way. The label faces the direction of the airflow. Unless I've made a blunder - I pointed mine at the PSU in order to cool it down. Facing it the other way would act as an exhaust by pump the heat out. I don't know which way is correct. I tried it the first way round and a lot of air was pumped out. I don't remember the fan being that active before so I've reversed it round so its pumping air in from outside, at the PSU and around the case - out by the hard drive.
Maybe someone can post me a message as to whether I've got this part correct.
I found that when I used the SilenX fan and the silicon mounts - I had to pull 'em hard. Incredibly - they didn't break and created a really tight and snug fit. I'm impressed. I pulled my so hard a dimple appeared inside the external surface of the mount.
The funny thing is - now my previously reasonable sounding internal hard drive sounds LOUD!
I'm going to replace it with a seagate.
Oh yes - here's some further information about my installation. I spoke to an electronics engineer before I started and he told me that the power supply unit contains a capacitor of 400 volts. Capacitors store volts even when the item is unplugged from the mains. This could really hurt at least. However, its also worth noting that the contacts are under the motherboard and since we're not working in that area, the engineer suggest it shouldn't be an issue but still something to be warey of.
If in doubt - wear rubber gloves.
And as Roonsmits rightly said, "Forgot to mention, I take no responsibilty for any problems that might arrise when doing this "
Same here.
This is just my personal account of how I changed the fan, I'm not suggesting you do this or follow my steps.
Cheers,
Andy