Pip appeal

Hi any help would be appreciated, I have an on going problem with my knee and I'm waiting on a knee replacement I've already had 2 operations leaving me with a very limited range of movement and degenerative aurtritis in my knee. Had pip standard mobility awarded but I didn't think this was the right level of support and couldn't understand why it was awarded . I've filed my appeal and received all the paper work having read it my original gp medical report for some unknown reason says I have full range of movement and with Physio can be resolved. Having gone back to my gp and seeing a different doctors he apologised and wrote me a letter stating my problems and difficulties with movement and pain ect.
In the dwp letter to the tribunal service they are completely dismissing my gps letter and asking it to not go to court because of the original statement.
I don't know what to do or why the original gp letter stated it, I've had a tibial platau fracture and two major knee operations and problems for the past 7 years all that are documented. What is the best way to deal with this. Please help