In these circumstances, I'm not aware of anything in the rule that
states that I must use a non-reservable train(s) from York to Leeds.
Any thoughts?

John

I remember that maybe 10 years ago journeys issued on the long tickets
(do these still get used except for Eurostar, or is it all credit card
size now?) listed your services with reservations, and then "suggested
service" for non-reservable trains.

Re the Durham-Leeds ticket, I wonder if Durham ticket office would do
you a reservation on a TPX from York to Leeds, by you manually showing
them your tickets?