After you paid your 3rd bill, when your fourth bill is generated it shows as a credit on your account.
You can then call customer services and ask for it to be refunded to your bank account.
Numbers to call are in the link below.
Please be aware that customer services are working with fewer staff due to isolation and distancing rules and are extremely busy so it may take a while to get through.
We have reports the best time to get through is just after 8am.
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here