Well this one is pretty straightforward really, it does exactly what it says on the tin. Although, in the spirit of honesty you can attempt to secure email codes from a number of sites for somewhere around this price, peace of mind can be a wonderful thing...especially if that promised code never arrives in your inbox.
These days, you're generally looking at somewhere in the region of £32-37 to bag yourself a year's supply of online Xbox goodness, but just type in the code 'bill20' (all lowercase) at the checkout and you're looking at a tasty sub-£30 price.
Remember to use the code bill20 to get it at this price.
See you in cyberspace.
Thanks to HellRazer at HUKD for the tip-off.