Did some digging and it seems I did owe this last payment, therefore as I'd cancelled the direct debit I have sent a card payment.
In my defence it looked as though the app had stopped working entirely with the "Sorry to see you go" banner but I eventually worked out I could still navigate to the details of the bill and it was for April - May so I can't argue I was still a customer for that period.
If a Mod reads this feel free to delete the thread.
