There is a wiring diagram here.

What do you mean by the numerals 4 & 8 in "I would have a look in at the LH switch connections 4, black/purple 12v from horn and 8 black ground for horn button and LH indicator."?

I will check out the left hand control connections and their earth.

Could the problem be caused by a poor earth from the horns to the chassis?

I really appreciate your help here NIbiker - thanks mate.

Cheers, Richard
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09 America, Staintune Pipes, K&N, Breathe, Hagon Nitros, AI & O2 removed, tune 20184 (modified), MTX-L a/f gauge