I understand what you are saying, and I know that the issue asked by the OP is now finished, but in this particular instance there really is a flaw to your argument. This is what the SET booking system says about the journey:
As you can see, it says nothing apart from "London Underground" for the section between Stratford and Paddington. Now, you cannot even do that journey on one train, let alone a suggested train, or as you seem to insist a booked train. There just isn't a booked train. The passenger is expected to make their own way from Stratford to Paddington. This can be accomplished in numerous ways, among them being to change at Liverpool Street. Now between Stratford and Liverpool Street tickets are inter-available meaning that the Underground will accept NXEA tickets and NXEA will accept Underground tickets. So, the only possible issue is whether you can use a reservable train to form part of a connecting service. I'm not convinced that you can't. If someone purchased an Underground ticket and decided to use NXEA between Stratford and Liverpool Street would NXEA turn round and say they can't use this train because they don't have a reservation for it? I don't think so. In this case the ticket for the journey is effectively an Underground ticket via any chosen route between Stratford and Paddington which might include Stratford to Liverpool Street. Why is this a different Underground ticket to one which is purchased for a local journey in London.
In summary, I do understand your arguments where they relate to journeys that the planner will always give specific trains for. We've discussed where a connecting train from say Torquay runs in front of the booked train between Newton Abbott and Exeter so could the passenger change at Newton Abbott instead of Exeter as per the reservation. In that instance I will cede to your letter of the rules and agree that you can't, although I think most guards would in practice allow it. But in this specific case, where no specific trains have been listed, I don't see how it is not valid for the ticket to be used in the way that the OP suggested. Neither the ticket, nor the reservation coupon(s) nor any valid itinerary says what train(s) to get between Stratford and Paddington. All it says is London Underground, and for the section between Stratford and Liverpool Street that can also include NXEA if the passenger so wishes.
I have no arguement with the point that, as an inter-available route, Stratford-Liverpool St. is valid by train, DLR or tube if a valid ticket is held.
However, you must agree that if the intent (and 'booked journey') was to go to Liverpool St. on that train they would have a reservation for it. As they do not, it is clear it was not the booked train and is therefore not valid for it.
If the train was non-reservable, (as the London Underground is) the exact train might not be indentified and in those cases any service that can reasonably have fitted the time period could be considered the 'booked train'.
....There is absolutely nothing in the public domain to say you cannot continue to Liverpool St. The idea that you have to get off that train and get another is, in my opinion, absurd....
Your opinion is irrelevant. The journey plan is agreed to when the ticket is bought, deviation from that is "a change of travel plans", which is subject to an admin fee and possibly a further fee depending on tickets available for the 'new journey'.
If the passenger had requested, at the time of booking, to remain on that train to Liverpool St., would you argue they could disembark at Stratford for the tube connection, even though they have a reservation to Liverpool St.?
....There's a myth that you cannot use "reservable" trains on unreserved legs. However this claim is absurd, defies common sense, is unenforceable, not documented anywhere....
Utter rubbish.
Dictionary.com said:
myth [mith]
noun
1. a traditional or legendary story, usually concerning some being or hero or event, with or without a determinable basis of fact or a natural explanation, especially one that is concerned with deities or demigods and explains some practice, rite, or phenomenon of nature.
2. stories or matter of this kind: realm of myth.
3. any invented story, idea, or concept: His account of the event is pure myth.
A deviation from the journey plan is a change of travel plans. Travel where a reservation would have be given, but has not been, indicates a change of travel plans. What happens when there is a change of travel plans is documented (see below) and therefore is not myth.
freedictionary.com said:
Adj. 1. unenforceable - not enforceable; not capable of being brought about by compulsion; "an unenforceable law"; "unenforceable reforms"
A change of travel plans incurs an extra fare to be paid, this is documented (see below) and can be enforced.
Dictionary.com said:
document [doc-u-ment]
noun
1. a written or printed paper furnishing information or evidence, as a passport, deed, bill of sale, or bill of lading; a legal or official paper.
2. any written item, as a book, article, or letter, especially of a factual or informative nature.
3. a computer data file.
verb (used with object)
5. to furnish with documents.
6. to furnish with references, citations, etc., in support of statements made: a carefully documented biography.
7. to support by documentary evidence: to document a case.
Changes to travel plans are listed in the Advance fares conditions and should have been made clear at the time of purchase.
....Of course, we've had this all before. My view has not changed.
As with anyone else, you are entitled to your own views, even when they are wrong.