I can confirm Alex's memory of the initial reason for the Tristar purchase although a lot of people invented a lot more reasons as time went by. The wing pod option came later (or not as it turned out). Having been to several meetings at Cambridge I can also confirm that Marshalls do not come cheap. On asking for NVG compatible electro-luminescent lighting for the line-up lines we were quoted one million pounds per frame and that was in 1982.
Not only that, we only got coffee and biscuits, never a meal.