Volkswagen Polo sheared cylinder head bolt

I recently had a breakdown where a bolt inexplicably sheared off the cylinder head cover (RHS Front). This caused water (steam) to escape and the car to rapidly overheat. VW Roadside Assistance towed me to VW dealership who now tell me it will cost £2000 to fix(complete engine strip down). They have spoken to VW who decline to offer any goodwill. Surely this is not a common fault on what is supposed to be 'reliable' Volkswagen according to the advertising guff. Cylinder Head cover has never been off and car has always been serviced at that dealer. I've had the car from 9 months old.(80,000 on clock - motorway commute miles)
Any similar problems out there. Any advice would be appreciated.
Sorry if I've posted this twice.