Worth a link (it's on another thread too - thanks, Moonraker!) but 28 pages long.
http://nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdfHere are two paragraphs which come up here from time to time -
but please download and look atthe full conditions surrounding them before you rely on them - there are caveats and other conditions and I am not providing legal advise here!!!"You may start, or break and resume, a journey (in either direction in the case of a return ticket) at any intermediate station, as long as the ticket you hold is valid for the trains you want to use. You may also end your journey (in either direction in the case of a return ticket) before the destination shown on the ticket. However, these rights may not apply to some types of tickets for which a break of journey is prohibited, in which case the relevant Train Companies will make this clear in their notices and other publications."
"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."
I think that latter means that you can buy a Chippenham to Didcot season ticket, and individual Didcot to London tickets, and then travel by ANY train and not just those which actually call at Didcot, doesn't it? I used to live in Wiltshire, work a couple of miles from Didcot, and have occasional journeys into London ... wish I had known that one ...