The conditions require you to take "appropriate" connecting services.
See
this,
this and
this, to provide just 3 links out of perhaps a 3-figure number of threads discussing this problematic issue, for which we'll never get agreement among everyone.
The rules for your ticket used to be clear, simple, and easy-to-understand, stating:
Valid on date, GNER train and seats shown on the ticket, plus connecting trains as
required.
That was clear to me that the GNER train was compulsory (yes, even the seat... but this is rarely enforced) but connecting trains were not.
Then came "simplification" (or, in reality,
complification) and this wording was lost. The current wording is:
If the 'Route' also states 'and Connections' travel is allowed on appropriate connecting trains where shown on the ticket(s) or other valid travel itinerary.
It
could be argued that any train that shows on a NRE journey planner itinerary (which is quite customisable) can be considered "appropriate", but not everyone agrees with that interpretation.
I would argue it would be absurd for a passenger to be forced let a TPE train go in your circumstances which then results in being delayed, when the preceding trains would, by the definition I think most people would agree on, be "appropriate".
Although not part of the T&Cs, thus not forming part of the contract, the Knowledgebase (iKB) says you may take alternative non-reservable trains (in this case Northern only) but not reservable trains. A copy of this document is in our
Fares Guide in PDF format.
You need to go back to TPE and make it clear you were following the instructions of EC staff and you want TPE's policy. You could refer to the Stevenage-London exception example and ask if Leeds-York for a journey to/beyond London, is in the same category? It would be absurd if it wasn't.