Hi there,
I am pretty sure that a new law states that a manufacture can no longer refuse warranty because of where you go to get your service done. As long as the service was carried out at the right time and the correct service was carried out, and you can prove this of coarse with an invoice from the garage, then theres no reason for them to turn you down. Im not 100% sure on this but im quite certain im right. Goodluck and hope this helps.

George