Your old supplier will issue a final bill within 6-8 weeks of you switching.
This should be clearly labelled as a ‘final bill’ or ‘closing statement’.
If you believe that the bill you have received is not your final bill, please contact the supplier and request for them to look into why they are continuing to bill you.
It may be as simple as the old supplier having not registered the switch on their system, which they will be able to correct.