Gap cover refusing to pay for root canal deeming it specialised dentistry...

I had a root canal and Genesis my hospital plan did not pay anything based on on some technicality. Gap cover did not pay as Genesis did not pay. Genesis asked for more detail from my Dentist and reviewed their position and refunded som of the bill hence the "gap". In the meantime the Gap Cover demanded that I provide proof that the root canal was not for "cosmetic purposes" this was then provided and then they again refused to pay based on the fact that they deemed the even "specialised dentistry" . I am entitled to 2 x basic dentistry that identifies root canal as basic dentistry. The dentist did a root canal in her rooms and no specialist was involved. I have opened a dispute but now I am being ignored...