Watch for the dealer hanging on to your order for a number of reasons, could be that they have no build slots left for this month(or next etc) or they sometimes put the orders in to suit their own selling figures, maybe they have exceeded this month and are hanging onto a few to give next month a startup boost, hard to say why but they should have told you before you signed for the car if they didn’t intend to order it straight away!
If you get no build number then contact Motability and get them to demand one for you, at least you’ll know what the score is.