Just to add to what @gliders has wrote which really suns everything up to a T. You could approach Motability and ask if you were to take another car on the scheme would they agree to an early termination if/when a more suitable car for your needs came along. Assuming your car ends up a write off as you en vivid that is. Unfortunately there is not a lot more you can do, other than asking and trying to get a more equivalent hire car similar to your Merc or one that would suit your needs better.