iPhone 5 proximity sensor randomly not working

[Simonskiii]

Hello

my proximity sensor is working fine for calls, turns the screen off when the phone is brought to your face, and under a camera you can see it IR red glow of the sensor, but in the screen brightness options i have no option for auto brightness.....so i know the sensor is working, how do I get the auto brightness option back?

I have tried restarting the phone several time but to no avail :-(

Thank you in advance, Simonskiii

 

[Simonskiii]

Further Info

Just to add more randomness to the pot, the infra red light is actually on when the phone is sat on the home screen (not in a call basically), and if you shine a torch on the sensor the brightness level does creep up occasionally...however it doesn't go back down, and the setting for auto brightness still is not there...

I am very confused