Just a final update on this, I went to one of the main tube ticket
offices at KX a few days back to sort out my unresolved journey, and
the system wouldn't let the nice chappie resolve the journey for me,
nor indeed remove it, or indeed modify it in any way at all. Which is
a bit rubbish, as LO don't have a ticket office at Euston, London
Midland do, and don't care one jot.

So he ended up giving me a new Oyster card with the correct amount
preloaded, which could get expensive for them if they need to do that
to resolve LO journeys!