Just wondering, given that HSTs never had to couple / uncouple in service, how much dwell time is the 800 (5 car) coupling, adding into service? Would this outweigh the time savings that GWR stated?
Some - but a lot of the cross-bristol services which are planned to divide used to sit at Bristol TM for a while to fit into a standard path one side of Bristol or t’other, so on ordinary journeys you probably won’t notice. The “headline” journeys to which the time savings refer likely won’t have splits/joins as they’ll likely be peak hour trains.