I've done a little searching and found three batteries that would suit your needs. Phil has already linked to two of them and Simon has alluded to the third.
£20
£25
£30
You do get what you pay for, so the more expensive battery is better. The following analogy might help with understanding why...
Capacity: 4000mAh vs 4500mAh. Think of this like the size of your cars fuel tank. A bigger tank will drive your car for longer. So you will have a longer flight with a 4500mAh battery.
C-Rating: 20C vs 30C. Think of this like regular unleaded and premium unleaded. A higher C-rating will give you a little more power at full throttle which means you can throttle back a little bit and this will be kinder to your battery.
As others have pointed out, the connectors supplied with batteries do not always match those on the controller. This does not really matter because connectors can be changed easily. However, do not attempt to do this without seeking advice first. A common mistake is to cut off both the positive and negative connector at the same time and this risks the wires touching which at best will ruin the battery (instantly) and could even lead to a fire/explosion.
