Ignition Contact Drum Problem?

[TooMany2cvs 65]

I've got a bit of a funny going on here. Every now and then, the 205's flattening the battery - semi-randomly, in that it might be totally flat overnight, might be fine for a week or two between use. I don't think the battery itself has taken too kindly to this, plus a few episodes of headlight-related muppetry, so I suspect whatever's going on is being highlighted by a lack of capacity there. The alternator is very recent, and it's charging just fine.

 

But I think there's something more going on. Add in that the heater fan has become increasingly reluctant to do anything at all - then will suddenly start up mid-journey - and I'm wondering if the contact drum for the ignition switch is becoming flaky, sometimes failing to switch some circuits on, sometimes failing to switch others off. I've not started to dig yet, and I don't have a wiring diagram handy.

 

Thoughts? The car ('90 1.1 pikey-spec) now has 160+k miles on, so I can forgive it if that is the issue... but will I be able to locate a replacement...?