One on the "to do" list looked at today, been meaning to do this for a while now but anything involving pulling bits of wheel arch liner out tends to get put on the 'back burners'!
Finally got to it though & one of the wires had rusted through to the connector for the horn.
The brown one I cut to remove, the other one was fubar'd right against the connector so there was no way on earth I could repair it like that.
Took the little bleeder to bits,
Ended up with the above.
Didn't really fancy trying to solder onto what was left but looks like a repair kit is available - RealOEM.com BMW E91 320d Repair kit for socket housing, 2-pin
Will await good old Harry Fairbairn coming back to me on that one as can't find anything on line that isn't despatched from the USA at present.
With a bit of luck I will no longer be too embarrassed to use the horn by next weekend!