Sandown motors

I’ve had a problem with my Mercedes E400 Cabriolet since 2018. They can’t seem to fix the leaking roof. This last episode goes back to November 2020( 9 months ago) My car was taken back to Sandown motors. They did give me a loan car which I’m still using. My motor plan expired in July and I requested and extension on my plan as the car has been there so long. This was refused. Not sure what I should do as they can’t seem to fix the complaint. Having the car in the garage for 9 months seems crazy Gary Treger