This finally forced me to use the ATW App which is actually just about OK. 6 returns for £3.25 instead of £3.90 but the out and return tickets can be used separately which can be useful. Only hassle is making sure you've got enough battery left in your phone at the end of the day, and finding that elusive barrier with a scanner on it.
It also means that you're never without a ticket now they're really cracking down on people...