Upgrade your Xbox Live account from Silver to Gold on the Xbox Live Marketplace with a 3 month membership for only £7.50 - 50% off the standard Marketplace price.
While, yes, 13 month subscriptions can be found across the internet for £30, this is a) less costly in the short-term and b) far more convenient than ordering and waiting, as it will take effect in only moments.
I assume this will be a short-term deal, but I don't know exactly how short term - although it does say "Upgrade today", and knowing Microsoft, this could well mean 24 hours only.
Couldn't see this anywhere on here, so thought I'd post it. Enjoy!