iphone 5 power off screen coming up unprompted

Hi, having a really weird issue with my iPhone 5; just came on today. Basically my phone will just randomly go to the screenlock screen whilst I am in the middle of using it (i.e. the phone cannot in any way be interpreted as being in sleep mode). Whilst it does this it makes the camera shutter sound (WEIRD) and then it will flash up the Slide to Power Off screen. I'll tap cancel (cue shutter sound again) and then we go through the whole rigmarole again. In other words, it's totally impossible to use my phone at the moment! Chuck in the odd screen flicker for good measure. Is the issue with the battery..? Phone out of year's warranty but still within 2 year mobile operator contract, so I should be able to get them to sort I'm assuming? Thanks in advance for any pointers.