It would only be possible to make an xx14 departure from Cambridge off an xx56 start from Ely if the combination of dwell times at Cambridge and Cambridge North totalled no more than 2 minutes.
As it happens Cambridge requires a minimum 1.5 minutes (never mind any attaching) and Cambridge North at least 1 minute, so 2.5 minutes total, minimum.
Even if you could squeeze it behind the XC, that would need minute-perfect presentation nearly every hour, every day to be robust south from Ely.
Yes, I'd say it is probably fair that only having 18 minutes from leaving Ely to leaving CBG with a CMB stop does feel very tight - especially as going into platforms 7 or 8 at CBG, as the 'expresses' tend to do nowadays, is rather slow. And once you go to 19 minutes or above you do, unfortunately, clash with the XC
Also, as mentioned above, every other hour there is the Ipswich->Peterborough train, due to arrive Ely at xx57, which therefore also clashes.
Given that though, it is rather odd that the consultation timetable had it down as 16 minutes, which isn't just a bit out but quite considerably wrong. Doesn't say much for the use of consultation timetables if they are going to have such fiction in them
Ok, so I guess this isn't going to work unless you can rejig the paths over Welwyn (looks like you theoretically could a little, but goodness knows what knock-on effects would result) or change the XC timings (ditto).
So, instead, how about switching around those that start at Kings Lynn and those that start at Ely? The Kings Lynn could take the xx47 path from Ely, and it doesn't matter if it has to wait a while at CBG as it needs to attach more carriages there anyway (at least until they finally extend the platforms north of CBG). And the Ely starter could take the current xx17 path south of CBG, therefore starting from Ely at say xx24, restoring sub-70 minute journeys ELY->KGX.
As a useful side-effect, this would reduce overcrowding on the xx17. Currently there's been a 24 minute gap in services to CBG by when this arrives, and is only 4 carriages, already loaded with people from the Kings Lynn branch. If that arrived at xx47 instead, only 9 minutes after the GA Norwich, there would be fewer people trying to get on it at Ely and CMB. An empty 8-car starter would be significantly more comfortable to be the first service out after the southbound 'gap' (whereas currently at xx47 it tends to be rather lightly loaded, and a bit of a waste of capacity that would be much better at a different point in the hour).
Downside is that doing this extends the southbound 'gap' by another 7-8 minutes, and bunches the 4tph Ely->Cambridge closer than now.
I don't see any obvious infrastructure/timetabling issues with doing this, but I'm probably missing something
