Tim
The convention is that the horn is earthed through the steering wheel boss, so take that off - it should just pull out of it's Motolita, but may need persuasion. Inside you'll see a simple spade connector. Disconnect that. If the horn is now off, the fault is in the boss. If it still sounds then you have a short to earth between the horn and steering wheel. The horn should have two terminals - 12v in and 'to earth' out. Convention is that the earth out lead will be purple with a black tracer but this may vary. You can test which is 12v supply with a test bulb. There will also be a fuse for the horn, so if you can identify it, pull it out if required.
