Yes Will, you need to speak to Motability customer service about this as they are the ones who are in charge. The deposit side of things are between you and your dealer.
What happens is, when you place an order the dealer makes an application to Motability Operations. They then send you a copy stating it has been accepted and the Advance Payment etc Any problems, you go through with Motability. I have never cancelled or had any problems but you need to ring Motability to say that you’ve cancelled your order and that the dealer will issue your refund as you was not happy etc etc. You need to pester your dealer (I know he/she won’t be happy) about it in writing.
So in conclusion, chase your dealer up in writing that he will issue the refund, then straight after call Motability. End of the day, you have not entered a pin, you are within your rights to cancel.