Both the FOs and TSOs were converted from loco hauled mark 3s.
Grand Central have an imbalance in carriages, with six first class vehicles and nine standard class ones to allow for two first class vehicles in each formation, but of course only one designated first class coach is used. This means that at least one HST will have at least one declassified first class coach acting as standard class accomodation at any time, often more.
Grand Central HST formations have always seemed fairly fluid to me, certainly more so than those of the franchised TOCs.