In my humble opinion, I would write a to be signed for letter, stipulating that they have 10 days from the date of the letter to return your money or you will take them to the small claims court.
My understanding is your contract is with Motability, and the garage’s contract is also with Motability, as they pay the invoice. But I may be wrong, it’s a long time since I worked in law.
Time to play hardball with them, me thinks.
Bz4x fwd vision in silver