Further update – On Friday morning (5 days after I put my private plate on retention) the wrong number plate was still showing on our Motability account. I phoned Motability again, who agreed that it should have changed by now. They messaged the department that deals with this and thankfully by the end of the day it was corrected. I did 120 and 370 mile round trips in the intervening period but thankfully didn’t run into any issues with the Police!