For example, I have an advance single from A -> C, with a reservation from A -> B, then I am on a non-reservable service from B -> C. Firstly, am I right in saying I can take any non-reservable service from B -> C?
If you have an eTicket, your entire booked itinerary - including non-reservable services - will be clearly stated on the eTicket and will be encoded in the barcode. So it will be very obvious if you travel on a different service than booked and I would not recommend it.
If you have a paper ticket, depending on the format you may only have the reservable legs shown. In that's the case you would be travelling within the
Advance ticket terms and conditions if you take reasonable alternative services (i.e. not stopping off for an hour). You are tied only to the trains "shown on the ticket" (condition 4.1).
After all, you're not obliged to carry the original itinerary with you when you travel and so you might have no way of determining which unreserved legs you were originally booked to take.
Secondly, am I allowed to take a reservable service from B -> C?
I wouldn't
encourage that, but if you are travelling in accordance with the Advance T&Cs, i.e. on the legs shown on your ticket and otherwise "on appropriate connecting trains where shown on the ticket(s) or other valid travel itinerary" (condition 4.4) even with a paper ticket that doesn't list the non-reserved legs. It would be obvious that you were not travelling on your booked train, if a guard were clued up.
However there is an element of plausible deniability again - nowhere does the ticket explicitly state that the legs not listed have to be undertaken on non-reservable services, and if it doesn't list all booked trains I don't think you can be expected to know which one you were booked on, especially on higher frequency lines.
