I wasn't aware of it. The TOCs do like to keep these things secret.
FCC restrict
return portions of CDRs between 1630 and 1901 (inclusive) although the Any Permitted CDR to Peterborough is restricted between 1555 and 1901 as this is set by EC and they have different rules (they either co-incidentally chose 1901 as the end time or they colluded. My money's on the latter but who is going to take them to court? So they get away with it). SVRs set by FCC are restricted between 1730 and 1830 (both portions), while return portions of SVRs set by EC are unrestricted and for outward portions restricted 1559-1745. SSRs set by EC are restricted between 1458 and 1858. Nation
Simple!
ATOC claim that all these tickets count as "Off Peak" type tickets, this new name "[simplifies] the choice, allowing you to choose your rail ticket with confidence" and "The new fare names describe when you can buy or use your ticket, making it easier for you to decide what ticket suits your journey."
So, as all these tickets are called Off Peak, you can immediately figure out when they are valid, as the name Off Peak describes when they can be used. Anyone can work out all the times posted above simply, and confidently
For a laugh:-
http://www.nationalrail.co.uk/times_fares/simple_fares.html
And, seriously, anyone have a few thousand quid to take them to court over blatant lying, misleading the public and entrapment?