Found the fault! I just attached a wire to the lower part of the steering column by means of a clamp or tie wrap and run the other end to an earth and hey presto the horn works fine with ignition on. I think it was a dodgy earth under the airbag? There are plenty of good earth points near the bottom of the column, just make sure that the new wire does not foul any other moving parts and give enough slack to allow for full steering movement. I will worry about a proper repair if the airbag ever has to come off. Hope this helps anyone else.
Terry potts, October 2012
