I thought you were obliged.. But after my Honda went out of warranty and I took it to my usual garage (a lot cheaper, and more reliable than a main dealer) I was told the warranty would be honoured as long as the car was serviced by a "reputable" dealer and the service was carried out to the manufatures spec.
If I were you, get in touch with Citroen and get their reply in writing. I am sure the rules regarding this cover all manufacturers. Perhaps others on this site may be able to enlighten you more.
Good luck, should save a small fortune.