basically ring the motability grants team they will send you the form, you fill it out explaining why your injury means you need an automatic, you may need a doctors letter it all depends. in my case as i get a war pension because of damage to knees and hips it just got straight through. some times it involves a bit of to and fro but its worth the hassle because in my case its been a one hit thing.
each time i now renew a lease it happens automatically. your allowed one grant every three years. i lost out on one changeover as i ended the lease early as i needed a higher car.