This seems to be an exclusive deal via 3G but you end up purchasing directly from 3 Store.
It's 4GB for price of 2GB so I suspect after 12 months your allowance drops back to 2GB
Be aware this doesn't include any personal Allowance or Feel at Home.
Personally, I think the extra £3/pm for 8GB which includes Hotspot and Feel at Home is better value but this may be a good deal for anybody who's got a strict budget of £10/pm or more than happy with 4GB.
Direct link http://www.3g.co.uk/sim-only