I’d certainly be informing Motability, especially if you still have gearbox, handbrake issues and rattles, requesting termination on mechanical grounds.
If those are now sorted and it’s just the bumper, I’d still inform Motability, as they will get it soted alot quicker than you.
Write everything down prior to contacting them and go from your list everything that has been wrong with the car, whats still wrong with it and what you want from Motabilty perspective, an mechanical termination or it repaired.