They did say to contact them if I'd taken a different train to the one they'd assumed I'd travelled on. Generally speaking on the GN side I think it's relatively easy to work out what train soeone's travelled on from Stevenage:
Touch out at STP - clearly voa Thameslink
Touch out at Kings Cross - journey around 40 mins will be a GN stopper
Touch out at Kings Cross - journey time under 30 minutes will be LNER
What if someone waits 15 minutes for an LNER train? The only way they can work it is to say GTR trains take a minimum of n minutes (allowing for being told to run non-stop to makes up time) - if it's over that, it's GTR, under LNER (and if they take too long, it's GTR
There will be some edge cases like the non-stop peak extras that operate from Kings Cross. I'll have to give this a try sometime but given my experience with getting KeyGo to work properly is poor I won't hold my breath...
Bottom line is that PAYG (as implemented in this country) works on a station to station basis - you're travelling from A to B, the fare is X: doesn't matter which train or which route, you make an assumption and that's it. The only way you do otherwise is by 'marking the card' (however - an OSI, pink oyster reader (only for a lower price!), on train inspection,...) of those travelling otherwise. It's a different paradigm and the railway has to deal with it.
GTR might claim it's only valid on their services but I'm not sure if this can be enforced. GTR set the inter-available fares between Stevenage and London and consequently are not allowed to create a 'GTR only' fare. I don't know if a PAYG system changes this, my view is it shouldn't.
I understand contactless will come to Stevenage in Phase 2 which will render KeyGo all but redundant. I seem to remember it being mentioned on here that LNER are involved in Project Oval as far as Stevenage.
To some extent there's a question over whether it's 'GTR-only' or 'not-LNER', and bearing in mind that it's a different medium, not a different fare.
If LNER are part of the inter-available offering, and Oval PAYG is being mandated on thet bit of the railway, then LNER will have to accept it - which means they get involved, or have to suffer the consequences.