The inbound service, 2C77 from Cardiff, arrived at PNZ 32 minutes late, only 2 minutes before the return to BRI» , 2M77, was scheduled to leave. Would 15 minutes, or thereabouts, be the minimum turnaround time for a 5-car IET▸ ?
Purely as an example of what’s possible, from real time trains, I found a 5-car IET formed service arrive at Temple Meads from Worcester, and then depart back towards Worcester in a shade under five minutes. I wouldn’t be surprised if that involved a fresh crew taking over the train mind.