They probably don't need them all to start with - some are for the added Oxford services when that opens. Are some perhaps to replace the Banbury LH set? In that case they could keep that.
Neil
What usually gets forgotten is that as well as the Oxford services (which are mainly already in the timetable as Bicester and Banbury terminators and therefore don't need much extra stock), there's a separate platform lengthening project going on at the moment to allow them to run more 9 car trains between Banbury and Marylebone.
As you suggest it is possible that may also involve replacement of the Banbury LH set as well, but I haven't seen that explicitly mentioned anywhere.
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Prepares to be shot...
I believe that SWT 158s mainly run in multiple, and as TPE services currently use trains with 1st class, I believe that 11 Northern 156s should have been transferred to SWT in return for there 158s, and SWT should be told to diagram 158+159 wherever possible, reducing services no longer containing first class.
I assume you meant to write '156+159' otherwise that doesn't make sense as you've moved the 158s elsewhere.
SWT 158s don't mainly run in multiple, about half the daily diagrams are single units, and most are services with advertised first class.
156s and 159s in multiple won't be pathable as 90 mph trains to fit between EMUs on the fast lines between Basingstoke and Waterloo. West of Basingstoke I doubt they'd manage to keep the current timetable either - which on the face of it isn't as big a problem, as 158s are not normally diagrammed west of Salisbury.