To put things in context, let’s pretend that the MML is operated as a half hourly service of Sheffield - London - Nottingham.
With a reasonable layover at St Pancras, let’s say that it gets into Nottingham 4h15m after leaving Sheffield
So call it a nine hour round trip (in theory you could do Sheffield - London in 2h and London - Nottingham in 1h30 but that’s not including any layovers, plus half the trains need to run “slow” to maintain the frequency at Beeston/ Long Eaton etc)
A half hourly service with a nine hour round trip would require eighteen diagrams
Let’s assume that we’ll need to have at least 10% of trains spare at any time (maintenance, accident damaged etc), so we have four idle at any time
Let’s say that one unit is a “hot spare” at St Pancras and one “hot spare” at Leicester, so that leaves 27 trains and 18 diagrams
(I’m trying to be conservative here by assuming six trains aren’t diagrammed to run services, similarly I’m trying to use a duration - nine hours - that doesn’t sound too tight and recognise that even trains with engines under 80% of carriages might not be able to run as fast as they’d like if there are freight paths/ flat junctions/ bottlenecks into stations like Sheffield)
Half of the diagrams can therefore be ten coaches long.
So I don’t think that there’s too much to worry about in terms of fleet size based on the current requirements. And a single 5x24 810 is equivalent to the standard class seats of a seven coach 222 (despite a significant number of EMR services being just four/five coaches - something that the 360s to Corbyn has helped slightly, admittedly)
Obviously if they get stretched into things like daytime extensions to Leeds/ Lincoln then we might have problems, given the way that TPE’s 185s were horribly stretched once they were expected to cover the WCML to Glasgow/ Edinburgh. Whilst I understand the reasons for the specialist 24m carriages, it does leave EMR a hostage to fortune in future once the production line closes, but I suppose they can always order some 9x26m 80Xs to work alongside the 810s.
I’d like to think that we’ll reach “now that the MML has been fully electrified to Sheffield, what do we do with the 810s?” before we reach “how do we order some additional self powered trains to augment the EMR LDHS fleet?” though!