Go to My Account/Bills & payments, and this should show you the breakdown of how the £50 has been charged.
If you see nothing there, then you likely have a duplicate account, which trawling your emails may reveal details of.
If you can’t resolve this from the above, then talk to Live Chat, at:-
Get in Touch (all the options, and the best place to start):-
On a screen with big buttons all around, you may find the link you need is under a tiny green arrow, so be vigilant.
Live Chat and Send Us a Message:-
On Live Chat, when you can get onto it, keep your answers short, sweet and prompt, or it will time out on you.
If the person you are talking to doesn’t seem able to understand your issue, ask to be escalated to the next level of help.
The £11.99 per month charge relates to a NowTV Cinema Pass, where a £7.99 charge currently refers to an Entertainment Pass.
Have you been paying two £11.99 charges on the 2nd & 7th of each month, where if so this would suggest to me that you have two accounts with a cinema pass on each account.
Or is one of your payments on the 2nd or 7th of September a £7.99 charge which would be an Entertainment Pass ?
Do you have an NowTV Entertainment Pass on your account by going to My Account > Passes & Vouchers whilst logged in on here ?
Also what playback devices are you using ?