The spark plug on the middle cylinder is shorting.

Put new plugs in, car runs OK for a few days then starts coughing which steadily gets worse. End cylinder plugs are always fine, middle one gets a streak down the white outside ceramic where it has been shorting. Problem is not the plugs. I have tried two different makes of plug. A plug runs fine in end cylinger for 1 year, move to middle cylinder and is shorting a week or so later. Engine management light does not come on. For those not familier with Corsa there are no HT leads as the coils sit directly on top of the plugs. I cannot see any damage to the rubber socket for the middle plug.

Would some kind soul who knows why this is happening be good enough to enlighten me?