Quick update for anyone reading this in future: Both alternators supplied by the motorfactors were identical in terms of mounting points, pulley offset, etc, the only difference was the back of the unit and the amperage output. Oh, and the lower output one was 1.5kg lighter than the other, so naturally i'll be using that. 
I noticed in the pictures posted above that there were spacers missing from my mount, which i've now found and everything is just dandy. Thanks again to Tom and Paddy.
I noticed in the pictures posted above that there were spacers missing from my mount, which i've now found and everything is just dandy. Thanks again to Tom and Paddy.