Off-Peak Singles and the outward portion of Off-Peak Returns are valid for travel on the date shown on the ticket and until 04:29 the following morning. If the journey cannot be completed in this time, the ticket may be used to continue the journey on the following day. Unless otherwise indicated in the relevant restriction code, time restrictions apply as from the initial origin station on both days. The appropriate restrictions for the actual day on which travel is being undertaken apply (for example, it may be that if day 1 is on Sunday, no restrictions apply, but on day 2, the Monday-Friday restrictions apply). All travel must be completed by 04:29 in the morning after this second day. Please note that break of journey is not permitted on some journeys, as detailed in the ticket restrictions.
http://www.nationalrail.co.uk/times_fares/offpeak_conditions.html