Anyway, I noted that when driving straight the horn seldom worked. However turn the wheel a bit it was okay. Today I pulled off the wheel half expecting the contact to be dirty. The solution was a bit different. The little brass contact that is spring loaded to provide contact had slowly worn away, but there was still a 'lip' that would hang over the edge of the contact ring. This 'lip' prevented full contact because of an earthing plate behind the brass ring on the column when the wheel in in neutral (?). Anyway, to solve the problem all that is needed is to ensure that the bottom of this brass contact is level. Problem solved and now works fine. There is no need to remove the wheel either as the tube almost jumps out after the horn ring is off.
Hope this helps somebody.