Where an Advance ticket specifies a service for part of a journey, but leaves an open ‘&Connections’ for the rest of the journey, what is the position if the specified service covers part of the rest of the route (e.g. It duplicates the connecting service’s route for part of the rest of the journey)? Is it acceptable to remain on that service for a longer part of the route-effectively to treat that as a connecting service- and join the remaining connections from a station further along (which would be the same TOCs and services for the connecting journey as would have been the case from the specified station)?
You have to use the services for which you have reservations, and then use "appropriate connecting services" (compliant with any TOC or route restriction printed on the ticket) for the rest of your journey. It might well be that continuing to travel on the same service forms an appropriate connecting service, in which case you can do exactly that.
There is an internal staff briefing which purports that you must only take non-reservable services for legs where you don't have reservations but I'm happy to say that it's a load of bunkum which has no basis in the NRCoT or the Advance terms.
If the TOCs wanted to introduce such a restriction then they would need to change the terms, and also make it possible to find out whether or not a service carries reservations far more easily than they currently do. Hint - whether or not seat reservations can be seen is not the test!