Not expecting a lot of heat but may help some; this is down from £29.99 (RRP). (Next cheapest is £17:84 on Amazon. Orange £26, three £30 etc.)
BUT in store (HMV Canary Wharf) I've seen it for £9.99 and they had quite a few on the racks.
Feel free to vote cold if you know where I can get a cheaper power pack for an iPhone 4.
£14.99 on Amazon, seems to be expired on HMVhttp://www.amazon.co.uk/Logic3-Power-Sleeve-for-iPhone4/dp/B00445Y7PQ