Hi Philip, the onset of the GDPR legislation means it is very difficult to obtain historic information of a personal nature.
The registration number of AGG 7 is most likely a personal / cherished plate as date related plates were already in use by DVLA at that time. The number assigned in 1999 is most likely its original assigned number that was never issued on a plate as the personal plate was retrieved from the car.
Have you tried looking up the MoT history at DVLA? This may provide some clues as to where the car was located during its lifetime - try searching using both numbers.
N6 JMX