More likely a continuity error, the cars usually ran on false plates, the Capris had several plate changes and one was even taken back to Ford and resprayed in a newer colour to match the ones in the showrooms. The two capris in the final series VHK12W and VHK11W were used much earlier on in the series with older plates. The RS2000 also did not use its genuine plate, this car is currently being restored.
I think the first plate is false as the "JN" in the reg is normally found on the Ford press fleet cars used in the show. The reliability issues were in series 1 when Doyle had a TR7 , Bodie a Sprint and Cowley an SD1.
Cheers, Paul