This link will allow you to add one additional month onto your existing Xbox Gold Subscription.
It will re-enable the auto-renew, so make sure you turn this off afterwards.
It will work on Xbox 360 or Xbox One accounts.
Kudos to Phreak09 on Whirlpool for this one (http://whrl.pl/RezOqk)
Enjoy!
Worked perfectly, cheers. :)