My dealer told me if i take the private reg off my current car and put it on retention, then i can just take the retention document to them beforhand and in the 1st sept, rather than registering it with a new number issued by DVLA they can just use my private reg. Apparently they do the registration process on line so i assume there must be a way of attaching a private plate at the time so it goes straight on the V5. Sounds a bit ominus to me. As all cars with private plates still have their original number in the background on file then the car would still have to be issued with this new random plate number first i would say.