Well, I can't really help, although I believe the tube started running to LHR in the late 70's and also recall a London map from about that era that had "West London Air Terminal" marked on the page, near Earls Court.

As noted I never used the service, but did use an equivalent in China in the late 90's and it was a bizarre experience. Utter waste of time IMO as we seemed to do the same things twice - both at the City Terminal and again at the airport proper!

As for my 1st TATL, that was in the 70's in a narrow body DC-8. Route was Newcastle-Shannon-Bangor-Atlanta, and the idea of a family holiday across the pond was so rare, not only did my folks get cards from their friends before we left, but I'm told that my primary school classmates all went into the playground to wave at the plane as it took off. (School was under the flightpath).

Those of you who b*tch & moan about an in-op IFE system really haven't lived until you've spent 14hrs on a plane that, erm, doesn't have an IFE system. Kudos to my folks for keeping us entertained on the journey!