Terrible! Its a difficult one because overall you would be waiting a whole year for it. On the positive an extra 4 months would mean you get the car you want, a face lift model and hopefully some gesture of goodwill to reduce the AP or discount off extra’s for their mistakes.
I would be tempted to put in a complaint and speak to the manager and say you wont be leaving a good review on the motability website under the dealers and then make a decision after that. Theyr’re probably unbothered about you cancelling it as they know someone else will take the car.