My horn decided to stop working, so as per elsewhere on the site, I unplugged the negative terminal on the battery, left it for a while, then took the airbag assembly off the front of steering wheel and unplugged the odd connector on the back of the airbag itself (which might've been a mistake?).
I got the horn module out, cleaned up some corrosion on the circuitboard and soldered a fresh wire, put it back together, reassembled everything, and the horn now works correctly.
The problem now, is the airbag light is on. Should the system even have realised that I'd unplugged it given that the battery was unplugged at the time, or have I genuinely mucked something up?
I've got an ODB2 cable but I don't have the red adapter cable to try and do a reset using MultiECUScan, I've had a hunt but haven't been able to see the adapter available separately anywhere.
(Edit: Aha - Found this thread, DIY goodness: http://www.alfaowner.com/Forum/alfa-...ing-xenon.html
Given that I fixed the horn in order for it to get a warrant-of-fitness (MOT in England I think?) - having the airbag light on is going to undo all my hard work.