This is a customer community forum. not O2.
You need to contact payment management.
Details here Guide: Coronavirus Community Help and Support
To pay bills, ask for payment extensions etc, you need to contact the Payment Management Team.
You can reach the Payment Management team on 0800 902 0217.
The opening times are:
Monday to Friday – 8am to 8pm
Saturday – 8am to 6pm
Sunday – 9am to 6pm
Best time to call is around 8.00am
Could be the final payment after cancelling which is due on the regular payment date.
Looking in your MyO2 would confirm this or not.