I had a Polo that did that - centre push on steering wheel.
Took the wheel off and investigated the connections. Eventually found where the horn earth lead was making connection where it shouldn't and added some extra insulation. Took several attempts mind! Not before some poor chap had alsmost banged his head on his bootlid as I was reversing out of a slot in Morrisons car park one evening...
If it's a cloumn stalk, I've no idea - only ever had them fail, no horn...
