How does the ticket inspector know which is the booked train.
By looking at the reservation coupon(s).
Most people would agree that if a train is "booked", you get a reservation coupon.
I have an advance Cardiff Bay to Romsey ticket validity bookedtrainonly but the journey needs 3 trains Cardiff Bay-Cardiff Queen Street,Cardiff Queen Street-Cardiff Central and Cardiff Central - Romsey. The only information about trains on the coupons is the seat reservation from Cardiff Central-Romsey. Therefore presume I could use any trains for the other 2 legs.
That would be a logical interpretation, in my opinion, although some people disagree. There have been numerous debates on this issue on the forum, a search should reveal them.
The law does state that where the rules are unclear, the interpretation that is most favourable to the customer should prevail.
The old airline-style tickets used to say "Suggested service" and give details of the trains that were 'suggested', the new-style tickets do not mention these services. I believe the word 'suggested' is self-explanatory. It may be possible to get the old style tickets from cetrain travel agents, I was surprised to see some examples used by Sunderland supporters just a couple of weeks ago (they bought Durham-London group advance tickets).