If it is an "Anytime" ticket, "You may start, break and resume, or end your journey at any intermediate station along the route of travel." it says at www.nationalrail.co.uk/times_fares/ticket_types/46544.aspx so it looks as though for that kind of ticket you don't have to buy more tickets.
Also for offpeak and SuperOffpeak, it says at www.nationalrail.co.uk/times_fares/ticket_types/46590.aspx that "Break of journey is allowed on the outward and return portions of Off-Peak tickets unless otherwise indicated by a restriction shown against the ticket's validity code."
You may start, break and resume, or end your journey at any intermediate station along the route of travel on Off-Peak tickets unless the ticket restriction for the journey you are making does not allow it. If you intend to start, break and resume, or end your journey at any intermediate station, please call 03457 48 49 50 to check if it is available on your specific journey."
If I were you, I would phone than number whatever the ticket type and ask a human being.