The problem is what's "irresponsibly late in the day".
I'd rather they changed the wording so it was 100% clear what was meant, personally. One of these must IMO codify it.
1. One overnight break of journey is permitted at the passengers' option. (In which case I'd just go the whole hog and show validity as two days)
2. Break of journey is permitted overnight only if it is not possible to complete the journey in one day. If it is possible, passengers must begin their journey early enough to complete it on that day.
3. As 2 but with the addition of ", unless using a Sleeper service."
If it's (2) or (3), I'd suggest just getting rid of the Off Peak tickets entirely on those flows, replacing with Anytimes at the Off Peak price. Such journeys tend to have unrestricted Off Peaks anyway.
If it's (3) it must surely be possible to make it clear by requiring tickets to be endorsed on board Sleeper services for a second day of validity.
Neil, I can't say I agree with you on this.
(1) will be very unlikely to happen, and (2) and (3) are both extremely anti-passenger.
To give you a real life example. Once I planned to get away with a friend after my Friday afternoon tutorials to Newquay for the bank holiday weekend. If I left Leicester after my tutorials it would not have been possible for me to get to Newquay that day without paying for a taxi from St Austell. My friend lived in London so I just stayed with him overnight, and we went together to Newquay on Saturday. All this was done on a Leicester - Newquay Super Off-Peak Return. Under your proposal, I would have either had to leave my students uncatered for for their two sessions, pay over the odds for an Anytime Return, or buy two separate tickets. Why should I be penalised for doing something that is entirely reasonable?
(Of course, they could just re-price the Anytime Return at (Super) Off-Peak prices, but you know as well as I do that it is not going to happen without an entire recast of the fares system.)
The current situation is not perfect, but as I mentioned earlier, I don't think there is a problem with the current system in this particular respect, and I get the feeling that we are proposing solutions for something that is not actually broken.