In case I missed it elsewhere, from the Motability website news section (updated 12th Jan 2023), the £750 New Vehicle Payment starts at the end of February 2023:
We’re increasing this payment to £750 from the end of February 2023. We hope that giving you this amount back will help you make the most of your new car or WAV.
If you already received a New Vehicle Payment of £250, you’ll get an extra £500 and we’ll start to pay this from the beginning of March 2023. This will make your total payment £750.
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