@Anonymous User
Does it definitely show that £9.99 was for the entertainment membership?
https://help.nowtv.com/article/how-do-i-view-my-payment-history
Head onto live chat by clicking “chat online”.
https://help.nowtv.com/article/how-to-submit-a-complaint
Or if you have screenshots of the offer you can also alternatively send an email to customer support. Address is on the below picture.

FYI that I do not work for NOW, just a NOW customer trying to help