They have offered me an alternative but it is a different transmission and engine size/type than the one I’d requested. It’s better than a kick in the teeth I guess, but should I accept it AND still do all of the above (in the hope I will finally get my requested motor, or should I just accept it and say nothing?
NO not if its not the one you want and plan on eventually getting the vehicle you requested as it will not happen. So hold tight. The only way to this is speak to mobility and ask if you took the car the dealer has offered, would you be able to replace an order for the car you wanted. Motability will probably say hold on to your current vehicle untill the vehicle you order comes back in stock. So essentially you will stuck with something you don’t want.
If you take the car the dealership has offered you, you are in reality ceasing any blame upon the dealer, as the dealer will argue the case is closed because you took a car they offered, regardless whether it was the one you wanted or not.
You are in a mess through no fault of your own. My advice is not to take the car offered, because it is not really what you want and follow @BigDave advice. I would also carbon copying motability in any correspondence to the dealer.
You are also free to cancel and order with another dealership.