Apart from the fact that the dealership are wrong to not refund the deposit as it is them that broke the contract, I cannot see why motability cannot contact the dealership on your behalf to point this out to them. I know motability say not to give a deposit but some dealerships insist on it to place the order. The fact is the car you ordered and paid a deposit for does not exist and your deposit must be in the dealership accounts and should be easily refunded. You definitely need to take this further.