Given the focus is on cost cutting and fleet reductions does GN actually need 20-30 extra units and if so for what services as it's a significant increase on the GN fleet size.
The answer is maybe. GTR have managed to deploy the 387 fleet in such a way that they have managed to reduce the GN 700/0 requirement from 8 down to 4, though the flip side of that coin is they have had to increase the GN 700/1 requirement from 0 to 1 (which nominally seems to represent the unit released from Littlehampton).
So it wouldn't take too many more Electrostar units to eliminate 700/0 from the GN side altogether, ignoring Welwyn-Sevenoaks of course. Especially as there are currently 387s having refurbishment and ETCS works.
Whether it might be possible to ditch 700 from any driver traction cards as a result I'm not sure, though this is only going to be of marginal benefit given the similarity to the 717 fleet.
The current situation compared to 2019 is that the peak Baldocks are still missing (6-8 4-car diagrams), plus roughly half of the peak Peterborough extras (at least 6 to 9 4-car diagrams). One can ignore the evening Peterborough and Baldock services as they would be resourced from the above. So to reinstate all the above we'd be looking at 12 to 17 diagrams.
Is there demand for these services? Again the answer is probably maybe, especially on Tuesdays to Thursdays. However is anyone going to want to pay for units to be doing one trip each peak? I suspect there will come a time when the current service is insufficient.
The other variable is that there's a significant amount of slack in the 717 fleet. This is being utilised way below what was originally envisaged. ISTR they are now 100-mph capable (is this now signed off?). I don't think there were actually any plans to utilise these for outer-suburban services, but it could certainly happen.
I tend to agree with the view that 30x extra units for GN would be excessive, so were it to happen it's a fairly safe bet that some 387s would be moved south of the river.
There was a proposal to alter the GN timetable so that the Baldock peak services extended from/to Cambridge, replaced by a Letchworth to London stopping service. This seems to have gone quiet, though elements of these proposals have appeared such as the Alexandra Palace call on the 2Cxx services. I know there was some kickback locally at the prospect of the Baldock starters being extended to Cambridge, as local users liked the fact that the Baldock services offered the prospect of a good number of empty seats without having to endure a journey on a stopping service, it also provided places like Knebworth with a fast London service. The difficulty was always that reversing at Baldock was difficult, really the Thameslink Programme should have provided a central turnback siding here. With housebuilding in the area, it probably won't be too many years before there is some need to reinstate these services in some form.