... and just to clarify: although the DD payments stop after the March payment, that season has by then been fully paid so the season ticket is still valid for the remainder of the season.
Isn't that what I said? As you rightly say, though, the club will confirm the date you need to let them know by. Obviously, it needs to be before the first DD payment for the following season - which I thought was April to March.
Cancelling at the end of the season carries no penalty, as it is a yearly renewable contract. If on DD, you will need to let them know in March that you don't want to renew, and your DD will stop. I did that this year as an easier way of moving seat (cancelled my ESU season ticket on DD, then...