How much do they know ? Chain stretch due to Longlife oils degrading and the tensioners will stratch to take-up the slack. They can only work soo much and if you hear a ratlly, mechanical noise when you start the car in the morning (for a couple of sec. while the tensioners fill-up) then your tensioners are either blocked or the guides worn. If it slips its a engine out job- i have seen 2 engines do that...not cheap at all ! There is a vag buletin wich explain how to change the chain guides in situ, not easy but doable but many garages will pull the engine out so they can time-it properly- not chap in labour costs...