Just a couple of things to mention, as one who has bought a voucher or two with invalid codes:-
1) Now TV will not help you if you bought a voucher with an inactive or invalid code, you have to go back to the retailer to activate your voucher or supply a valid code.
2) You said that you've just got around to setting up your Now TV account, vouchers, passes etc. are only valid for 1 year from date of purchase, and from some other retailers only 6 months. Therefore, if you took over a year, from purchase, to set up your Now TV device and account then the codes are now invalid, and you will now have to purchase new passes, vouchers or gift cards.