Dave Cross wrote:

But before I do, can someone please confirm that I'm not going mad. You
can buy travelcards with prepay credit can't you? I know I've done it
before. Or was that just a glitch in the system?


Not a Travelcard, but I bought a single ticket using the Prepay credit
on my season Oyster at Euston Square at 7pm last tonight.

Correct amount was deducted.