In the past it was always easy just to ask the dealer to cancel and immediately re-order the car with Motability. Give your dealer a call and ask if there willing to do that but as Jojoe said you could lose your place in the queue.
Ask the dealer if there willing to change the order as it takes them only minutes to do but with folk paying over the odds to get a new car quicker the dealer could speak with a forked tongue and tell you they’ve changed the order but sell your car to a new punter at a higher profit then tell you the car is taking longer to deliver than we thought.
Personally the way things are today for £300 I’d let it go after waiting a year myself for a car to arrive with still no definite date for delivery I would have given my dealer £300 12 months ago to get the car straight away.
With what Jojoe said about a refund you could just ask Motability if there willing to lower the AP as a good will gesture in view of having a big heating bill coming in soon as that would be no lie and If they say no I would leave at that.
Remember though when ringing MB it will be MB Operations your talking to so expect the worst outcome.