From what it seems all 377s will be with southern and the GN 387s
The GN 387s aren’t going anywhere in time for May, if at all.
This has been through so many iterations it’s become hard to follow, however from what I gather as things stand at the moment the following will happen:
* SN 455 withdrawn from May timetable change, replaced by a reshuffle of SN’s existing 377 fleet.
* SN 313s withdrawn in time for December timetable change, with SE 377s displaced to cover, and some of their work covered by retained Networkers freed up by the remainder of the 707s.
Where the 379s might fit in to all this is if there is a need to provide additional SN and/or GN capacity. Presumably the stored 465/2 could also act as a backstop for SE for as long as they remain stored.
If we reach December and, as it appears, all three of the 313 365 and 455 fleets have been removed from GTR with only the SE 377 as replacements then things will be very tight for any capacity uplift.