Maybe a timing chain slip and caused a bent valve or another timing chain tensioner failure. It could be a number of things lots to rule out.
They should be able to do basic first with the engine in. Check chain, stick a camera in to view piston and valves to rule some things out before any larger dismantle.
The cars are getting on a bit now and unless it was under a vw warrenty I think it wiser to go to a vw specialist rather than a main dealer.