Basically all the current carnet are 'Inter Available' and can be used on either East Coast or FCC services between Stevenage and Kings Cross not Peterborough as the carnet is not currently available from there.
Just to be clear: are you saying that the combination of season ticket plus carnet ticket I mentioned in post #10 is allowed or not? If not, why not?
I can't see how it wouldn't be valid. The carnet ticket is inter available and is, for the purposes of splitting, just a normal point-to-point ticket. A season from Peterborough to Huntingdon is clearly a season, so the combination must be valid.