Thanks for replying Angus
What's got me really confused is that exactly the same fault occurred last year which was apparently fixed by my local Citroen dealer by replacing the comms unit.
This time however, they say its not the comms unit, but might be the BSI unit.
Maybe there is some info I'm missing, knowing that simple things seem to be made more complicated than they need be these days.
Do you know where the engine and passenger fuse boxes are, I've looked for them, and fuses F8, F17 with no look.
I'll start tracing back from the horn as you suggest.