I’d suggest filling in the complaint for on Motability website & getting thru to someone who can do stuff. In my exp the call centre staff are usually polite enough & try but have been hopeless when I’ve spoken to them. When I filled in the complaint for I got one person sorting it from start to finish
From what you’ve said you have to give the dealer a chance to fix it, they car has been there weeks we’ll that was their chance to fix it. Whatever the fault is is not the point you are paying fir a car you cannot use they’ve had a chance to fix they haven’t so tell motability that’s it. Tell motability if they don’t act you’re going to your MP if necessary.