Kingsley-Hughes, again, on an issue I’ve run into myself.
If you’ve noticed that your data usage has skyrocketed since installing iOS 9 then this may be a feature rather than a bug. Wi-Fi Assist may be to blame.
Wi-Fi Assist switches your iPhone to using your cellular data plan if you happen to be in a spot where Wi-Fi coverage is poor. This sounds great in theory, but if you’re not on an unlimited data plan then all this extra pressure on your metered data plan could result in a nasty shock when you get your next bill.