Buying train tickets

If it's what they call an "Anytime" return, you can break your journey. "Advance" tickets are different, they're only for a specific train.

Are you just planning to break the journey on the same day or over a period of several days?

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.

If I am reading this correctly, it looks like if you are not changing route, then yes you can break your journey with an Anytime ticket.
trainsupport.custhelp.com/app/answers/detail/a_id/4705/~/ticket-types-available-and-terms-%26-conditions
Hope this helps

Sorry, had not seen Elegran and grannyticktock had already posted the answer. The trials of a slow internet!

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.