From National Rail ...
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.
NOTE: Cross London transfers will not be accepted by London Underground during this extension period.
No limit on length of break?
I'm travelling from Melksham to Cambridge tomorrow, back sometime mid to late in the week, possibly with a stop of up to 48 hours in London ...
As I read it, I can travel from Cambridge to London on Wednesday evening on a super offpeak, then on to Chippenham in the middle of the day on Friday, and perhaps on to Melksham at 19:01. But I have to use the King'C Cross -> Paddington section on the Wednesday. Is this right?