Right, as I slowly work through the niggles on this sorry excuse for a car, I've come to the AC.

The diagnosis so far is pretty much "well, I know that bit is missing" but I dread to think what caused that whole gubbins to be removed.

Am I worth just getting a used compressor, swapping the belt thinger and giving that a bash, or am I missing something critical?

A known good compressor, the proper belt and a regas should do de trick IMO cos it sems that it was a clutch failure on that one