Do you know the changeover date the factory starting incorporating the digital dash as standard and the date your vehicle was built?
I would be emailing the dealer, telling them you are holding back on any payment for now, whils’t you seek some professional advice, as you disagree with the charge especially as you were told the digital dash is now included as standard and payment won’t be needed from a member of staff, which you took in goodwill. This will also give you some breathing space.
It’s worth speaking with citizens advice and/or your MP, letting them know whats gone on.
If you were to put a complaint in, it would have to be with the dealer as Motability will tell you to take it up with them anyway.