So much guff from people, it's unreal. You can buy OEM Apple batteries from quite a few places, and what's all this nonsense about warranty on a phone that is going to be 3-4 years old? Get a cheap one from eBay, or buy an iFixit kit for €24.99 (about £20), which includes the tools to do it.
Ah perfect, the iFixit kit is exactly what I was looking for. Trusted seller, decent price. Happy to do this myself so for about £20 this seems like a no brainer.