John – just complete the form as best you can. If you cannot provide a name, provide a short letter explaining the circumstances (i.e. leased Motability vehicle, not in your possession any more, name of dealership, etc.) and attach this to the form. Take a scanned copy (or clear photos) of all documents you complete and nip into the Post Office to send it back to the police – and make sure you get proof of postage (use 1st class mail).
The form is a ‘Notice of Intended Prosecution’ (NIP), bundled with a legal request for driver information from the registered keeper (using Section 172 of the Road Traffic Act), so John mustreply. If he doesn’t comply, he may be committing an offence and end up in court, getting points and a big fine. Motability cannot reply for him.
Whilst informing Motability and getting their advice isn’t a bad idea, John is/was the registered keeper of the car and is therefore legally responsible for its use – and, crucially, completing the form. Still, helpful to let them know, as they can note it on your account should there be a problem later.