So, onto the horn itself. Off with the grille and horn cover and out with the horn. Jumped it across a spare battery and it nearly blew my eardrums out - problem not there then.
That left the wiring and steering wheel mounted buttons to explore. Now, up until this point the car had been parked in the road so I thought, before I go any further, I'd better park it in the drive. As I was parking I tried the horn one last time . . . and, yes, you guessed it - it worked
After a little trial and error I established that the horn works fine UNLESS the steering wheel is within about a quarter of a turn from full lock in either direction, which obviously it had been when the car was parked in the road!!!
Is this just a quirk with my particular car or is that the case with all of them?