A/C

On Series I it is located where the AC pipes go through the bulkhead (at least for a LHD car, I have no idea on RHD). The bulkhead bridge is an oval metal part surrounded by rubber, with two pipes leaving it. It is bolted onto a similar part on the inside of the cabin which is a part of the heat exchanger, literally through the expansion valve assembly. What you do is (of course once that the remaining gas has been evacuated) to unscrew the two nuts that hold the oval part on the engine side of the bulkhead.

There are several cabin temperature control systems fitted to the various models.

What is "Climate Control" (on a P reg, 1996 XM TD 2.5) as opposed to Air Conditioning?

There were several 'climate control' systems fitted to the XM. In fact, IIRC the manual refers to them all as 'climate control'. Onseries 1 there was:

1) Manual
2) Semi-auto (looks very similar to manual)
3) Full auto

1.5) Later series 1 also came with manual but separate for driver and passenger side.