Details now promulgated on the main Motability website.
One thing that stuck out, as from detail posted above, is eligibility for the payment:
Anyone who is already a customer as of 31 December 2021 will become eligible for the payment when they take delivery of their next vehicle, whenever that may be.
so not just those existing customers changing cars in 2022.
Full details on Motability website here:
New Vehicle Payment introduced for Motability Scheme customers