With no sign of new or bi modes I can't see northern withdrawing any until the promised but not materialising second hand dmu's appear. From 507021 excellent list I make it expected are 5 150s: 5 156: 5 158s and 6 170s.
When any of these will actually appear I know not.
K
The latest edition of Modern Railways says January for Scotrail cascade and first pacers going off lease. First 195s are apparently on course to enter service December and Manchester to Preston electrification should be complete then too but I would imagine Northern want the month overlap in case there are problems.