Replacement Mobile Phone Batteru Problem?

[Irishsam]

I bought a replacement battery for my SE W800i as the battery thats with it is 3 years old and battery life is halved. When I got the phone it lasted 17 for hours of continuous music playback which I use for work. It now only lasts 8 so I have to charge it every night. So I bought the battery online from memorybits, Charged it lastnight, It lasted from 7am-11am on the radio which is awful. Memorybits says they will change the battery but is there actually any point? Unknown to me when I bought the battery its not an Original SE battery.