On GTR Great Northern Super Off Peak tickets to London are only available for travel on Saturdays, Sundays and Bank Holidays. The fact that a Saturday timetable (with a few extras in the peak) will be operating on the normal working days between Christmas and Bank Holidays doesn't mean that Super Off Peak tickets will be valid.
The real scandal is GTR not removing the evening peak restrictions. For the next three days we have a farcical situation with fares between Peterborough and London where the Off Peak Day Return Any Permitted can be used on ANY train yet the Off Peak Day Return Great Northern Only retains it's morning and evening peak restrictions.
Consider the 0726 Great Northern service from Peterborough to Kings Cross on 28th December:
The Any Permitted Off Peak Day Return at £33.30 is valid
The Great Northern Only Off Peak Day Return at £27.70 is not valid
Passengers making a day return from Huntingdon, St Neots, Sandy or Biggleswade to Kings Cross at 'peak' times tomorrow would be better off buying an Off Peak Day Return from Peterborough and starting/finishing short.
Then again, would we expect anything different from GTR....