The Plan also adds, following the above quote:
Reading between the lines, NR would appear to be considering the possibility of ~10m extensions to the 75m platforms at Flowery Field and Godley, so that 4x20m units could be used on the Glossop/Hadfield line.
You may jest, but I would guess that the acceleration and platform dwell time of a 315 would match or better the timing performance of a 323 on the Glossop/Hadfield line - and provide a significant capacity improvement.
That would be an irony if the trains that replaced the ex-LNER 506s in Manchester were replaced by the trains that replaced the (originally) almost identical trains on the Liverpool St. to Shenfield route.