A leaking water pump definitely is a "mechanical failure" and if the warranty insurer won't cover it, then it's the liability of the dealer who sold the car to you. You need to send a letter to the dealer principal of the supplying dealership demanding they fix it under the six-month rule, otherwise, you will have it fixed by another Audi dealer and sue them for the cost using Small Claims. Since it is a matter of urgency, give them seven days to respond. Send it by Post Office Special Delivery, keep a copy, and staple the certificate of posting to the copy so it becomes a ‘matter of record’ should you ever need it in the future See: www.honestjohn.co.uk/faq/consumer-rights/