T5, even with the 2 satellites, was never going to be adequate for all BA's growth needs. Hence their limited additional use of T3 which will probably grow over time. One proposal in the late 80s was for T 3 and T5 to be fully linked by a large cut and cover trench with underground people movers and backup walkways as well as transfer baggage belts and backup roadway for tug drawn trolleys so that in effect they were one terminal with 2 entrances,- one in Central area (T3) and the other in T5. One day something of that sort will happen but it would have bee cheaper and easier to do at the outset.