nothing to worry about with Motability or being the registered keeper, a paper trial proves different, I would suggest your vehicle was brought by the dealership and didn’t alter registration to keep it for resale as one owner/full service etc. It’s very common but no come back on you, it’s a standard letter sent out by dvla if the vehicle has be untaxed or unsorn as off the road for a certain period. If your vehicle had gone to auction then the paper work would of been dealt with, it’s a dealership/sales thing. No come back on you just dvla checking to see if they can charge for its downtime and different ownership, ignore it they will produce a new v5 in the new owners name as long as you don’t claim ownership.