Just to clarify, was it a super off peak ticket from Birmingham to London super off peak, and on the return leg, on a weekday, on a train timed to depart Marylebone after 04:29 and before 08:30, or after 16:00 and before 19:00 -- presumably the 0604, 0631, 0735, 1737 or 1837?
It's worth double...