I was incorrect on the first part, it's not permitted to break overnight on last day of validity of the return portion (edited in above post). The remainder is right though - the validity is until 0230 on the day after the last day of validity (25th in your example):
I stand corrected. We are actually both wrong.
Break of journey
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 08457 48 49 50 to check if it is available on your specific journey.
Where Break of Journey is available on your specific journey, if you wish to stop overnight or it is not possible to complete the journey by 02:30 on the day following the last day of validity, you may continue your journey the following morning. Travel must resume before 12:00, where the time restrictions that apply to the ticket should be observed and no further break of journey is allowed except to change trains.
NOTE: Cross London transfers will not be accepted by London Underground during this extension period.
As per NRCoC point 11, you may continue your journey if the validity expires during travel (i.e. you're on a train at 0230 on the 25th).
Do you have a source for this? AFAIK an Advance can be excessed to any single fare. If that fare is interavailable, the excess remains TOC-specific, which would be explained when the excess is sold. The excess is only valid with the original ticket so this restriction could be checked by any staff.
From National Rail website:
Changing the time or date of travel
Changes to time or date of travel must be arranged before departure of the first reserved train printed on the ticket, after which the ticket has no value and a new one must be purchased. You will need to present the ticket(s) and reservation(s) when you request a change.
Changes to tickets cannot be made on-board the train. If you board a train without a ticket and reservation for that service, a new ticket must be purchased. Depending on the Train Company you are travelling with, you may be liable to a Penalty Fare if you board the train with an invalid ticket.
The origin, destination and Train Company or route shown on the ticket(s) must remain the same.The difference between the price paid and cost of the next suitable fare for your journey is payable, plus a £10 administration fee per person, per single ticket for each change to a journey. If you change to a train on which a cheaper fare is available, the difference will not be refunded.
