George and Steve.
Thank you for the replies. I wanted to check the horn switch because the workshop manual said to do that. I found a good earth at plenty of positions on the steering column.
I've had the car 4 years and there has never been a 10 amp fuse for the horn in the engine bay fusebox. The connection is there but the fuse is missing. The horn has still worked nevertheless, until now. The wiring diagram shows 2 horn circuits, one of which is for the security system and has a 7.5 amp fuse in the driver's footwell. This fuse is intact and there is power at the fuse connection.
I also found power at the 10 amp point in the fusebox. With nothing to lose, I put a 10 amp fuse in and the horn worked. Perhaps only one of the horns is working, but at least the car is road legal again. I might investigate further if the rain stops. If I discover anything interesting I will post it.