Just in case it helps anybody, I have managed to sign up for the season ticket fairly easily.
A bit of background: I signed up for this last year and I was on a further promotional offer until I cancelled it earlier today. I tried to sign up to the season ticket but it wouldn't let me due to me 'already being on a promotion'. 'Cancelled' was showing in the sports section of passes and vouchers in my account, rather than 'Apply a voucher'.
It took five minutes on live chat for the operator to make "Apply a voucher' show in the Sports section of my account and from then it was simple.
Don't forget to update your payment card details if required at this stage.
I then followed the season ticket link from the home screen and completed the checkout process.
Just been on chat myself to get this sorted and was informed that chat has an auto disconnect function if there is no activity after two minutes. I was advised to type **** (that's not hiding a swearword, simply four asterisks) from time to time to keep the chat live. Sometimes the person you're dealing with has to talk to other departments to resolve your problem.