Right Gents
As promised, I have taken a couple of photos of dubious quality, showing the alternator slipping through the gap between the bulkhead (specifically the power steering pipe joints) and the driveshaft.
As I mentioned earlier, I took the alternator out without removing the bearing of the idler driveshaft, as people have advised that you do have to. I can confirm, with evidence that you can simply jack the engine forward on it's mounts to give you that magic couple of millimetres that you need.
The first photo shows the alternator going through the gap in the small access hole
The next two photos show the alternator sat in the space, below where it normally lives.

One thing that I will say is that the bottom bolt is easy, but the top one really got me, there is a spacer to balance between your fingertips while you push the bolt through, whilst being careful not to drop the washer, all the time you can't see a fecking thing. Then you need to tighten the nut and bolt at 1/16th of a turn at a time, whilst working against gravity, my arms still hurt 3 hours on!
Still, I honestly can say that it is not as bad a job as A), ones that I've done (cam belt and rear suspension arm bearings) and B) as other people on here have stated, and I am by no means a mechanic. What I would say is have a go if needs be, but ffs choose warmer weather!
Cheers for your help.
James