If you don’t get the car serviced motability will send you a reminder to get it done and say if you don’t you are in breach of contract.
it happened to me. I had the car booked in to be serviced. Took it up to dealer who said that car doesn’t need serviced and removed the service reminder. A few weeks later got the letter from motability to say I hadn’t got the car serviced!! I phoned motability who said it did need serviced and they have authorised it on the system and they would be ringing dealer to tell them that . Car was then serviced!!
So I would just give motability a ring and say service is going to be a bit late but it’s booked in.