Well,,, after a bit of a wait, i procured a second hand genuine 120 amp 2 pin alternator of the Bay with 2011 stamped on it so its a low miler.
I changed the regulator brushes for the new set bought prior, I was going to do the whole thing properly and change the slip rings and bearing but as it was in good shape with minimal wear to the commutator, I just striped the alternator down, thoroughly degreased it (Jizer) and threw it in the dishwasher for an intense wash,, it was a real grotty state. I think a previous owner had tipped a lot of engine oil over it or had a leaky oil filter canister.
I decided to follow the above method from Snagger and go in 'from the top'.
I followed a print out of the above posters method and as i have a rather good high-lift alloy professional jack and some decent axle stands it was a no brainer.
The weather today was excellent if a bit blowy but all went to plan.
Being able to raise and lower the engine at will made it all work very well and no dirt in my eyes from doing it all underneath.
The only help i needed was to push the engine backwards enough to allow enough space to extract the alternator up out of its hole, and then again to lower the replacement one in.
I had a couple of spare gates belts to hand bought of a member here and as the AC one was seriously shot, (166,000 miles) they both got changed as a bonus.
The whole job took 5 hours and although I said never again, i would do it the same way every time.