Oh my goodness! I wasn't going mad! I'm having to weed out a few weird ones where there's a specific route priced silly money to make you go the other route eg Hereford to Long Buckby charges £45.10 for a return via brum and £146.50 for "any permitted" route.
So turns out there's a bunch of uk railway ticket rovers/rangers (tickets which allow unlimited travel over certain areas) which are now cheaper than off peak single/return fares *inside* the rover/ranger area they cover. I'll be doing some data crunching and reporting back.