Robin,
you raise an interesting point, which has nagged me for some time. RACAL did produce a report on the SUPERTANS fitted to ZD576, but I have never seen it and as far as I am aware it is not in the public domain.
You may recall that the AAIB found that the there was a pre-existing fault on the doppler antenna.
My poor understanding of the system is that it 'blended' the information from the doppler and the GPS to arrive at the position of the aircraft.
It is yet another loose thread.
EG