@matty125 If you don’t have the commitment in writing then I fear you are relying on good will from the dealership. There is always room for manoeuvre. The line that many dealers use (“all we get from Motability orders is a small handling fee”) is hogwash, as each Motability order also counts towards their volume targets, which determine their overall discounts from manufacturers and bonuses.
Without me even asking, not only did our Hyundai dealer fully charge our PHEV, but they supplied it with a full tank of petrol as well. They would have still made a handsome profit on the car. I’d suggest a polite conversation along the lines of how disappointed you are that they had promised both the lower AP and the £500 contribution and if that doesn’t work ask if they can meet you half way. Good luck but if you aren’t successful try not to let it take the shine off your new car. You were happy to order it at the price you are paying and it’s a lovely looking car. The AP on our Tucson dropped £1,550 less than 2 months after we took delivery. Some you win, some you lose!