Im with EE but my problems appear to be hardware related.
After speaking to Apple at the weekend, its agreed to replace the battery or handset. I ran a diagnostic tool via the Support site (when booking a call) which indicated a hardware problem and when I spoke to the advisor, another one was run. I also advised that I had calibrated the battery and tried a fresh install and when I was then asked about screen brightness, confirmed that I have it on low as I dont like it bright!
Before upgrading, I suspect that battery life had been getting worse but hadnt thought much of it as it wasnt impacting on how I used it (i.e. I wasnt having to charge noticeably than normal) although I thought it didnt seem to be as strong as usual. In future, Ill be paying more attention!
As well as lower battery life, the battery left indicator is a lot more off than previous for example, when it went down to 10% yesterday, the iPhone almost immediately turned itself off.