Well after lots of wire tracing and pulling things apart, I gave up and threw it into a local place to have a look. K Motors (LR Direct workshop in Leyland).

They cleared the codes, and also took it apart. Cleaned everything up again and so far, the horn hasn't gone off. They gave it a clean bill of health and I drove away happy. We are a bit mystified as to what the root cause was, but that gremlin seems to have gone away.
 

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