I was in my closest Apple Store earlier in the week with a similar issue to you (my camera doesn't work at all). The genius said that Apple are aware of an issue, and its becoming more and more frequent.
I would book an appointment at the Genius Bar and you should set it swapped. Mine worked once, and since then, I just get a black screen.
With an issue like that, I would look to get it swapped sooner rather than later. After all, in this day and age, you would expect an £800 phone to have a camera that is reliable!
Just make sure you are sure you will be able to reproduce the issue when you get to the store though, as I had to wait a little over a week for my appointment.
Hope you get it sorted!