For those of you who have not learned to take Blind Pew's tales of derring do with a pinch of salt I have an alternative opinion on why the Tri 3b was introduced to BEA as a separate flight.
By not requiring the soon-to-be-retiring senior men to becom multi-qualified it gave management the opportunity to promote to the top fleet relatively junior captains from Vanguards who were the ones with lots of jet experience as RAF pilots and Comet co-pilots. They were also the ones who believed in training rather than checking.