Geoff, did you put your personal number on your car immediately you bought it, or was it previously issued with the 51 plate? If it had the 51 plate on it, this number will always be held by the DVLA and not used on another vehicle.
If at some time you sell your car and transfer the personal number to another vehicle, the original number will be re-issued to your car again.
I have a personal number and when I bought my new 172, the dealer got confused with numbers so I let them go ahead and issue my car with a 51 number and swapped it myself. When I had some warranty work done, I had to tell Renault what the original number was, as this number and not the one which is actually on my car, is the number held in their records.
The guy at the dealer explained to me that the 51 number would always be kept to go back on my car, should I sell it later and want to take my own number off the car.
Hope this helps.