Things have moved on, this is from the Motability website:
I’ve never even gave it a thought when I paid my last AP last year but it was with my debit card and the debit was £3,569-00 so its odd that MB state that “some dealers” will accept payments up to £1.000 when mine accepted £3.5k with no questioned asked, this was on hand over day.
Mind you this was after waiting for the car to be delivered after 15 months wait so maybe they thought not to get on my bad side any more than necessary. 😊