No I mean Manchester to Crewe and Stoke. You don't have to actually use a train on a route to prove it's unsuitable. Porterbrook have done all sorts of computer simulated trials to prove expensive modifications can be made to the 319s to make them suitable to replace 323s when they currently aren't suitable but no operator took up that offer.
Was that the famous idea to knock out the TSO and give the 319s a slightly better power/weight ratio than the 323s? You don't need a computer simulation to work that out, I've just done it with my calculator.
How expensive the modification would prove would probably depend on what happened to all the redundant TSOs. If the answer to that one is, they would be thrown away, yes, I suppose it would be an expensive modification.
You're forgetting extra services run at peak times
There are only two peak-time extras, both Virgin - 0627 and 0700 Picc-Euston. The 0627 runs eight minutes in front of another Virgin, and doesn't conflict with the stopper. As for the 0700, the stopper which, on the normal cycle would depart Picc at xx.46, instead leaves at 0632, gets overtaken by the 0635 Virgin at Stockport, then manages to remain comfortably in front of the 0700 Virgin all the way to Stoke. The stopper could leave at 0635, the extra three minutes are needed for the Levenshulme and Heaton Chapel calls.
and there's quite a lot of double track sections.
All my calculations have assumed that all of Stockport-Stoke is double track, with no passing places, and, indeed, there are no scheduled overtaking manoeuvres. Whether a stopper could be sidelined at Macc in order to be overtaken by a late-running express, I don't know, I haven't checked it out. I know there is a third platform at Macc, but it doesn't appear to get a great deal of use, other than in respect of terminating trains. Oddly, the third platform is not acknowledged on the National Rail station plan.
Virgin services delay Northern on a regular basis but Northern are required to fit their services around Virgin not the other way around.
I suggest that's simply not true. Taking the situation of the off-peak, there are two VT and two XC trains per hour in each direction. If those trains were to be scheduled at even intervals (i.e. 15 mins apart), there would be no scope for running a stopper, without arranging an overtaking move at some point. In fact the four expresses are scheduled to leave Stockport for Stoke at xx.16, xx.23, xx.35, and xx.43, leaving a 33-minute gap before the next, and therefore time to get the stopper all the way to Stoke. Clearly, someone has made a point of creating a space for the stopper.
In the opposite direction the equivalent gap is only 25 minutes, but it's still enough to accommodate the stopper.
If there's a high risk of a Northern service delaying a Virgin service the path won't get approval.
I'm inclined to suspect that an existing service would take precedence over a proposed, but I think all proposed path applications would be considered on their merits. Virgin could have presumably continued having their trains call at Congleton, but instead chose to go along with the idea for an hourly stopper to be operated.
I am just quite accustomed to Excel and its inner workings to be able to quickly drag calculations.