I just had the official Samsung entended battery arrive via Amazon:
http://www.amazon.co.uk/Samsung-Exte...739120&sr=1-22
for £18.90.
The new battery is rated at 2000 mAh, vs 1650 for the original.
Not much of a review, as i havent had time to benchmark in terms of battery life, but i thought it worth mentioning that my phone is now heavier, chunkier (it's a few millimeters thicker at what used to be it's thinnest point), and actually feels a lot more solid in the hand - instead of the rear being curved, it's now alot more 'straight' up and down, and the camera glass is also now protected whilst resting on a desk.
Frankly, any extra battery time is a bonus!