Assuming this is a 2015 and therefore the current B8 model of Passat, then this is very unusual and begs the question why it was for sale at a car supermarket and not a Volkswagen dealer. It could've been rejected by the first owner for a fundamental fault and as soon as a Volkswagen dealer hooks it up to his computer its history of problems will come up. So ask the Volkswagen dealer who has been trying to fix it. If he admits it has a history of faults then you could attempt to reject it to the car supermarket for having been fundamentally faulty on purchase using the ruling in Clegg v Olle Andersson, House of Lords, 2003. See: www.honestjohn.co.uk/faq/consumer-rights/