I recently purchased an easyjumper(TM) Carpow Multi-function Portable Power Source and Car Jump Starter 13600mAh.
There are many different makes and models of these things available, but this one had the best reviews on amazon uk. There are two alternatives that are available, one for 59:95 and one for 69:95 but it is only the quality of the case that makes the price difference.
Inside the case you get the power unti, a 12v cigerett charger, a mains charger, various adapters for different types of laptops, tablets and telephones and a set of jump leads to use when starting the car.
I have been using mine to charge uo my mobile phone and we have twice used it now to start Teres car when the car battery was flat. I was skeptical that it would start a car, but at the first turn of the key, the car started!
This is a piece of kit that I highly recommend to carry in your car, or in your rucksack.