I would let them keep sending you increased fines, let them waste all their money, issue you with a summons and then just say nothing to do with me here’s the proof. Let them waste all their resources all their time and money. This happens every day because motability are not informing the DVLA that they have sold the cars.
Very unwise advice. @Retro89 has done the right thing.
Agree with Glos Guy, if you do what is suggested by belfast4 you are simply building a whole lot more stress than you already have. Of course its Motability who should be dealing with it but!!