Hi --- Only that his confidence has been backed up by your own research and that he seems to be buying his stock at rock bottom prices, probably because he has some form of contract with Motability where he can buy at such low prices on condition that he takes most of the stock or a high volume of vehicles that are offered to him.
It is good to haggle a deal, but you have to admit defeat when you cannot find the identical spec car any cheaper from any other source, you could always try for the, "icing on the cake" and try get a few freebies thrown into his non-negotiable price such as extended warranty, full tank of fuel or free service to make the deal seem sweeter, if they are prepared to meet you partway.
At the end of the day he is standing by his prices and giving prospective customers the ultimatum.... take it or leave it ... or go and find it cheaper, which is his prerogative and a business approach that must work for him, otherwise he would not be standing so firm.? (unless he is a good Poker player).