Elliott
I am sure you are aware what to do but this is what Motability says re mods.
“You can order factory fitted options or accessories when you order your car, but you will be responsible for the additional costs. If you have any issues or require upgrades to factory fitted options or accessories, please initially contact your managing dealer to discuss this. If you wish to add accessories or make modifications to your car after you have collected it you must contact our Customer Services team for authorisation. Should such modifications be made without our authorisation you risk invalidating your insurance and you will be asked to remove them and return the car to its original specification at your cost.”