I'd be talking to VW customer service as it's the failure of the warrantied part that has damaged other components so the warranty should cover those too?
That was my first port of call. VW Technical suggested trying to remedy the matter with the dealer first...so with my son in laws agreement I went to the dealer, suggesting that a face to face would be better than my son-in-law formally sending the letter I had drafted (extract follows).
On xxx August the coolant light came on. My father-in-law who is a VW/Audi enthusiast was with me and as I am not knowledgeable about car mechanics he advised me to get it checked out urgently as we were going on holiday shortly .
I took the car to your service centre and advised your staff that the car ( a 2014 model) was covered by a VW warranty and had only covered about 70,000 miles.
Later in the day I received a call from your service centre advising me that there was a faulty water pump that would be replaced under warranty, however consequential damage had been caused to the cambelt and that it was contaminated and it would be dangerous to refit. I was advised that a complete new cambelt kit should be fitted (parts £203.80) In view of the strong warning not to refit the belt and to replace the cambelt kit in the interests of my family safety I reluctantly agreed to the replacement.
On speaking to my father-in-law he suggested that I ask to keep the parts. On collection of my car I was given a Cam belt. (No sign of other parts)
My father-in-law who has a long term relationship with x Audi and y Volkswagen has discussed the matter with their Service Manager – Peter x.
Peter showed the belt to 3 technicians including a Master Technician who suggested.
1. There was no reason not to refit the belt which they consider to be in serviceable condition.
2. Y VW would have advised replacing the belt alone whilst the water pump was being replaced at a cost of £20/30. All other parts in the Cambelt kit would not require replacing at only half the advisory mileage of 140,000 miles
As you will no doubt be aware I am feeling rather cheated by the actions of your service team and would be grateful for your opinion and hopefully a resolution to my concerns.
........
Upon visiting the dealer on Tuesday morning I met with a fairly hostile reception and they have agreed to refund the cost of the cambelt kit (minus a cambelt).. I am however genuinely concerned that such differing opinion can be obtained from Audi/VW dealers and that when a part is faulty and covered by warranty other parts allegedly damaged as a result are not apparently.