Thanks for the help.
I have had contact from a couple of crew members. Yet again, Pprune proves its worth for this kind of thing. Last year, thanks to Pprune I was able to connect a RN Lynx pilot with the Argentine Aermachi pilot that had been trying to find out who he was. The Lynx had been a hair's breath from being shot out of the sky when the Argentine pilot spotted his ship in the distance and went for that instead. He flew so close to the Lynx he got a look of shock from the pilot as he flew past from behind. He always wondered what happened to the crew he had spared that day. They have since met up a couple of times and are firm friends.
We're aware the mission left on the 20th, but it was still an important mission on the 21st, given its duration and close approach to the Argentine coast. The squadron logs are in Kew, along with detailed files on the LR SURVOPs of the 12th and 15th - but that is all. The one from the 20/21st isn't there.
But, thanks once again Ppruners, for the help provided.