Andrew, I cannot comment on the heater pipe issue, but on my 2009 Roadster the horn was as yours. The cause was poor machining of the steering wheel boss, so the centre would stick on the housing unless pushed evenly and dead center, impossible to achieve in the real world.
I change to a "slimline" steering wheel boss to give me 15mm extra room, it came with a plastic, not alloy centre and low and behold, the horn worked perfectly every time.
