As Paul says or
When you have a more accurate delivery date on your new car you could tax the mobility and get free tax on your new vehicle . That way you can claim a refund on any tax left when you hand the mobility car back.
or you can ask the dealer to hang on to the new car on the assumption it arrived closely to when your mobility lease comes to an end and then pick up your new car and give them the mobility car at the same time assuming they are Motability dealers.
What car did you decide to go for in the end @myself33