Follow this link:-
and work through the options until you reach the deeper options that let you either have a Live Chat with a NowTV support representative, or raise an email covering your issue, whichever means of contact you prefer.
NowTV will then be able to explain where you, or possibly they, went wrong, and more importantly no doubt, to establish your offer for the balance of £49.00
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.