Thanks so much. I went out for a walk and came back to find all these helpful replies. I haven't phoned yet but I shall. I now know it isn't such an unusual request. I do intend to break the return journey over several days to see DC and DGC en route. I will travel off peak and use my railcard.
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.
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.
I have bought train tickets for a holiday. I chose an open return so I could change the return date. However I now want to break the return journey twice. Does anyone know if I have to buy 2 or even 3 new tickets? The websites have no answers. I fear I will have to. I wish it was easier to buy and change tickets. No wonder so many choose to drive.