Well, it cost £700 but it's all working as it should now. The garage officially claims that the air-con problem was the compressor clutch slipping, and that the problem with apparent noisy idle and fast-idle always being on was down to air getting into the fuel line via a cracked fuel filter housing. I know the latter is crap because it was only cracked by my dad a short while before going in, and the fast-idle problem was there before long that while I was on holiday there. However, one cannot deny the fast-idle now cuts out like it should. And the air-con is working at idle.
So, one hell of a lot of money, no real answers, but it's all working.
Xantia 1.9TD Series 2 - Air-con odd problem FIXED
Moderator: RichardW