Paul have you ordered some dealer extras which could explain the £500 deposit, If you are have not ordered any dealer extras they should not have charged you any deposit.
Regarding the £500 you have already paid is there any terms and condition, if your deposit is not refundable were you made aware of it at the time.
Personally I would contact Motability straight away and would not pay anymore money .
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.