I already have all the latest updates, it was after them that the issue started.
I know it doesn't make sense but yes, when the screen is locked and the phone is idle, the buttons remain lit. I don't have a case so it doesn't seem to be a problem with the light sensor.
The phone hasn't been dropped or in contact with water. It has been well looked after so there is no physical damage. It seems to be a fault with the phone if no one else has any knowledge of this.
Looks like i'll have to send it in for repair!
Thanks for the replies