The way I understood it was that the DfT» dictated the '158s' go to the Northern and East Midlands franchise. All the former Wessex units left the FGW▸ franchise, except 158745/746, with the FGW complement being made up with ex-Transpennine Express '158s'.
Didn't Wessex Trains have 12x158s and FGW now have 16x158s, resulting in an increase in four units as well as getting the ones with the comfier seating type?
When the Northern franchise started West Yorkshire Metro kicked up a fuss because statistically some of the most overcrowded trains in the country were operating in the Leeds area. The first thing Northern did was move all the First North Western 158s to Leeds from Manchester in exchange for smaller 150s being moved in the opposite direction.
Northern then looked at getting additional Sprinters and were allocated four 158s from Central Trains. These were then subleased to FGW with the intention that they would be returned from FGW with additional units in the future. Before this happened DfT changed the plans so that a couple of the FGW 158s would go to
EMT» and Northern would instead centre carriages from EMT. They also decided that although 158s would go from FGW to Northern, Northern would have to sublease 8 158s to Scotrail as soon as that happened, where they've been ever since and look to be remaining there permanently.
Apparently some of the 142s released from Northern to FGW were for the purpose of filling in for overcrowding work and have since returned. A change in operations has put these units back in to use. The Manchester to Scotland service went from Virgin to
TPE▸ and TPE had to stop running a regular Manchester Airport to Windermere service to run the Scottish services, as they had to run the Scottish service with existing stock. This left a gap in the busy Manchester-Preston corridor and Northern have since filled that gap by providing an additional service on that corridor.
