PhilD wrote:
If you paid by a different method (cash, card, warrent), that would
give a different code. I think M is for cash ("*M*oney", perhaps?),
whereas X is for credit/debit cards.

Ah, thanks, that makes sense as I did indeed pay by those methods.

A red herring, then, so the mystery remains. I can barely endure the
suspense until next Wednesday when I get my next peak travelcard to see
if it's £12.00 again

Paul