This unique instruction was still around at the turn of the century.
It's
still there:
RMKS 12. Operating authorities are to notify movements carrying pax of Gp Cpt rank or above.
I recall going there in a JP5 from Cranwell once with my QFI. Leeming ATC asked us the bizarre question "Do you have any Group Capatains on board?". Quick as a flash, my QFI responded "Hang on and we'll have a look!".....
It was some bizarre instruction left over from the days of the infamous 'PCL', dictator of Leeming and loathed by all.
The Avtur in a Basset accident was at RAF Valley in July 1973. Double engine failure on take-off, XS783 of 26 Sqn didn't ditch in Caernarfon Bay, but crashed near the aerodrome, killing the navigator and badly injuring the pilot.