Quite a lot of tosh in the preceding pages about the de-activation and indeed removal of the T3 boost engine. It was never the case. The boost engine was seldom used in anger during the latter years of BA service, resulting in serviceability issues, usually a failure to start. Aficionados attributed the cause to inadequate fuelling during start due to lack of use.
An unapproved but reliable remedy was to purge the fuel line by removing the boost engine igniter CB and perform a start cycle until a nice cloud of fuel vapour appeared from the jet pipe (usually requiring a helpful observer - or the startled comments of nearby a/c) A blowout cycle was then performed followed usually, by a good start.
As a member of the crew performing the very last BA Trident flight, Zulu Mike, LHR to Wroughton on 28th Feb 1986 to join the Science Museum collection and I can attest that we performed a full power takeoff with boost and that the empty a/c went up like a homesick, if noisy angel.