As I understand it you do not have to use a main dealer, but (there's always one isn't there!) the vehicle must be serviced to the schedule and standard of the manufacturer.
That leaves an easy get out for the supplier to argue that the garage you used "under the arches" or at a budget franchise failed to use original parts of carry out all the service operations correctly.
Bearing in mind that the service is tax deductible and you don't want hassle with a vehicle you need for work I stuck with the makers outlets till the warranty expired.