Service and charge

My vehicle broker down and had to go in to BMW for diagnosis and quote for the warranty to approve. The quote was sent to the warranty company and repairs were approved as well as a oils service. BMW service provide took 3 days to send photo of the damaged part to the warranty company, repairs were approved and i was informed that there is a shortfall, upon questioning why there is a shortfall i was informed that i have to pay for diagnosis. I don't understand as they were given the job and we never had this issue with other BMW dealerships before. If they do the job is diagnosis fee not waived? my understanding is that you pay for diagnosis if you move the vehicle to another repairer. BMW South has forever lost me as a customer even though they are the closest to me.