Don’t pay and call motability. You pay them a whack of cash each month make the resolve the issue.
Stupid dealer you can’t let someone collect something then call afterwards for more money. Imagine going to a restaurant settling the bill then getting a call the next day, sorry we undercharged you you still owe us £100. You’d laugh at them.
Might also be worth talking to the manufacturer too but I would be making this Motabilitys’s problem you’re their customer they need to look after you