I am glad the thread is still running-as an 'aviation enthusiast' (I stopped actual spotting at 14) living right on top of LHR Tridents were the most common site or so it seemed.
Also the most common sound but they didnt stand out that much against the Caravelles (how could two Avons make more noise than 4 or so it seemed ) VC10s etc. The T3 was also easy to tell by ear because the boost engine made a very distinctive noise.
Re-reading this fascinating thread again today it brought back a question to my mind. Sometimes when Tridents lined up for take off ( and I remember this scene with them using the intersection on as it was 10R (block79?) as the engines spooled up a jet of white vapour shot out from below the engines-was this water injection or something else?
Wonderful and fascinating variety of sights and sounds in those days at Heathrow-no doubt why I am sitting here reading PPrune 40 years on.
PB