[GazYorkshire]

I had a similar problem with a Granada a few years ago.
On the underside of the steering wheel there was a brass or copper ring, which made the contact for the horn.
It was covered in a grey film, so I gave it a spray with WD40, gave it a wipe with a cloth, and that was it sorted.
Not sure if the set up's the same, but might be worth a look.