I’ve just extended my lease last week. I’m under 15k miles. Spoke with motability and they agreed an extension no problem. Took about 5 mins on the phone. Got a confirmation letter through the post yesterday.
As far as I’m aware your GCB increases by £150 for every extra year you have the car up to 5 years so £600 after 3 years, £750 after 4 and £900 after 5