why not just use the vouchers? i use the service for movies and pay for a four month pass from amazon for £29.99 much cheaper than the price they charge
amazon also do a 6 month pass for the package you have, also for £29.99
sometimes they are cheaper, managed to pick a 4 month pass up for £20 a while back but went back up to £29
https://www.amazon.co.uk/Months-Sky-...ds=now+tv+pass
pass from amazon = £29.99 6 month pass
month pass from nowtv = 6.99 x 6, 1 month passes = £41.94