slc3 wrote:
No, I went to a mobile phone shop on the high street, why would this make a difference? I can understand if the battery is not as good but why does it show 100% charged until I unplug it then just die?
Because you didn't get an authentic Apple battery and there is no way to guarantee an aftermarket part put in an iPhone. Even more difficult for you, since you allowed someone other than Apple or an Apple Authorized Service Provider to open and install non-authentic parts in your phone, Apple won't service the phone.
Your only option is to go to the service you had install the battery and see if they are willing to do anything for you.
You need to understand no one but Apple or Apple Authorized Service providers have access to authentic Apple Parts.