I had this exact same issue and scoured the forums for a solution to no avail.
I called O2 who offered to send it to Apple, which would take a couple of weeks 😡
Solution:
Book a Genius appointment at your local Apple Store. The guy had a very quick look and swapped it out for a brand new one. That I got mine through O2 didn't seem to be an issue.
Back your old one up first then restore to your new one. They never said what the fault was, I guess its a hardware issue.