apologies if you've already looked, but you may find some more detailed info here:

www.motability.co.uk/main.cfm

I imagine when you place the deposit is something you can agree with directly with the motability dealer and see what they say. I imagine most will work to a convenient solution.

Double check with Motability when they will stop taking your Motability allowance and when you have to give the car back... when you have that date as a definite then go and see the dealer you intend to buy from and tell them you would like your car to be delivered on that date...

Then keep your fingers, toes, etc. crossed that they will do that and not renage on you!

(You may gather that I take "delivery dates" from dealers with a pinch of salt!)