Good place to start is the fuse.

Thanks for the suggestions. My solution follows for anyone who might have the same problem. The horn is indeed situated behind the front bumper, which is massive and made of plastic. I was worried I might have to remove the whole thing, but luckily I only needed to extract the row of 4 plastic lugs along the top, just under the front of the bonnet. The top of the bumper could then be pulled forward far enough to just about squeeze in and access the horn. I unplugged the connector using long-nosed pliers, and used a test light to discover that current was reaching the connector when the horn on the steering wheel was pressed. So the horn itself was faulty. Huge sigh of relief. Used a 12 mm spanner to undo horn retaining bolt, but first looped some wire around the horn in case it fell down into the void. Again, it was a tight squeeze but I could just wriggle the horn out through the gap at the top of the bumper. Water was dripping from the horn and (when shaken) it sounded full of water - it was! Had nothing to lose so took horn apart (4 screws) and left it to dry in the sun. It looked pretty manky inside, but I put it back together with some grease around the edges to stop future water ingress (optimist). Connected to a 12 volt battery in the shed, the horn worked again. Refitting was a straightforward reverse of the removal process. Hope that helps someone :-)