Thanks everyone for the advice!
I'll probs do Advance. Yes, I am making the journey (don't ask why I'll suddenly be in Stevenage and going to Durham!). Shame, because I may not be able to plan that far in advance.
Perhaps a lack of split fares is a good thing. Was kinda hoping some
CDRs▸ * for different legs might be cheaper than a SVR* for Super Off Peak.
*Yes, you've seen Btline revert to the old fare system - but hopefully just this once!