The conditions of carriage (available from
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf ) State:
19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover
the entire journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit their use);
(b) the train you are in calls at the station where you change from one ticket to
another; or
(c) one of the tickets is a Season Ticket (which for this purpose does not include Season
Tickets or travel passes issued on behalf of a passenger transport executive or
local authority) or a leisure travel pass, and the other ticket(s) is/are not.
43. Help from Train Companies if you are stranded
If disruption caused by circumstances within the control of a Train Company or
a Rail Service Company leaves you stranded before you have reached your
destination and the Train Company whose trains you are entitled to use is unable
to get you to that destination by other means, any Train Company which is in a
position to help will, if it reasonably can, either arrange to get you to that
destination, or provide overnight accommodation for you.
As can be seen, clause 19 states that you are considered to be making one journey, and clause 43 states that in the event of canellation, delay etc, then the TOCs should get you home.