That's by far the most sensible suggestion I have seen so far on here.
Could you elaborate as to why you'd think thats likely? I'd suspect its highly unlikely.
Whilst I'm not suggesting a 156 wouldn't be a good fit for the job, NR are well underway with the 153 project and it would require a ground up redesign if they were to suddenly introduce a 156 - specifically thinking about the modifications to the underframe. If you introduce a new type, you'd have to start again with the mods, rather than following a set approach.
Then, of course, you'd have to start looking at route clearance. The 153s are slowly getting more and more clearance and have been modified to achieve this. If you introduced a 156, you'd have to start this lengthy process from scratch.
Given the 156s are heavily used by both Scotrail and Northern and would readily slot into one of these established fleets, I'd argue that, especially because of the PRM mods and modern interior the unit has, it'd find a natural home here. It's quite possible that, just like the tenders seen elsewhere in the country to re-lease an established fleet, this is all part of a process for SR to simply take the unit on a more favourable lease.
But all this said,
@43096 do you know something we dont?