An update for future records guys.
Turned out to be a faulty relay and the contact on the spring under the horn button was in upside down. This prevented it from having tension on it from the spring and hence stopped it from contacting the earthing ring under the horn button on the steering wheel. He also had a dirty connection on the horn itself.
Just a note:- Took off his left side dash pad looking for the relay, as that is where it is located on the 77. Bugger me the bloody thing is in the engine bay on the earlier models. Doohh!
So jolinari is the horn still ok after castle hill? Did ya clean that connector on the horn?