D365, the original suggestion came from me! Angel were touting this as we were negotiating the DA3 award. Someone at GWR heard about what we were up to, assumed it was going to be 365 trailers and put it out there but for Angel it was always going to be Angel 465 trailers or no deal.
A 5 car unit with tabled 2+2 seating - about 400 seats - would have been very nice for Cardiff-Portsmouth (and other places) but the HyDrive scheme was too much in its infancy for the DfT to go completely nap on it.
You could get a good mix of 2, 3, 4 and 5 car turbos that could have seen off the GWR cl.158 and all but a few GWR cl.150, which battery 465 units would have been my favourite to replace on the few lines for which a 23m coupled unit would not have worked.
You can only augment if you went HyDrive because the required power isn’t there otherwise. Angel have checked the structural integrity of the Turbos and they are good till at least 2030, with some relatively minor work in the meantime. Some calculations were also done to make sure their specified output could meet the required Portsmouth-Cardiff timings.
Since the DA3 award the money has got tighter and the HyDrive project has not gone as quickly as had been anticipated. There are other possible solutions out there now as well.