Well, by my reconing, Northern is going to receive 8 units for 'additional capacity', so...
These 8 units mean that they can have 16 extra center cars without sacrificing diagrams.
Take into account now that 7 units are coming in exchange for the 180 fleet, and four units for two 156s
Lets say that these 7 extra would be two perminant 6 car formations, 1 spare, this spare is well, spare, so that makes 18 center cars.
IIRC NT currently have 18 150/1 units and 22 150/2...
Just from the extras gained by cascade, they could extend all of their current 150/1 fleet to 3 car units. With no loss of diagrams, making their now 150/0 fleet more practical as no corridor connections on 3 car units is better than on 2 car units.
Add on 150001 and 150002, and the two orphaned toiletless cars. Add them into some of the remaining units at Northern and this would give them a fleet of...
22 150/0 3 car units...
Then the remaining 150/1 and 150/2 fleet, I'm loosing count but started with 22 150/2 units, and 18 150/1 units, 20 converted (10 cascaded units)...
The total cascade being received being, 7 units for the 180s, 4 units for the 156s, and 4 units for capacity, then the extra 4 units. Sooo...
Northern get 20 units lengthened, and gain an extra 9 2 (or 6 3 car units) car units and 2 3 car units.
20 units lengthened would leave 20 units with northern at 2 car length, with 18 potential centre cars sitting about, it could be that NT would like to have a fleet of 38 3 car units and 2 2 car units. Or a mixture of both. There are some routes that can only take a 2 car unit, but a lot of routes would have 3 /6 formations as ideal.
The joyous thing being that NT has plenty of 150/2s to chop up if they want, then can have a nice big fleet of 3 car units instead of 2 car units.
I wonder if the DfT will let NT reform the units as they see fit...
Correction, it's 6 units for the 180 fleet, not 7... sorry...
Maths again...
NT will have:
18 150/1
22 150/2
18 (6 for the 180s, 4 for the 156s, 4 extra, 4 subsequent extra) 150 units.
Currently there are 48 (assuming 2*150 for 180) units worth of diagrams to cover, leaving an extra 8 units that could be chopped up, so, with the addition of the two orphaned units, all of NT's current 150/1 fleet could be re-formed to make a 150/0 fleet of 3 car units. With the addition of 150001 and 150002, would make a reasonably uniform fleet of...
20 150/0
16 150/2 (for definite)
18 unspecified 150 units.
Would mean no more actual units for Northern, but a lot of them would be longer...
Would be the extra pacers for extra units. But NT doesn't need more units for more diagrams, it needs longer diagrams, and more can be longer with 3 car units kicking about.