How to stop iPhone draining data when tethered to iPad and 'doing nothing'

I have a 10GB data allowance on my iPad (mini 2). I tether my MacBook and iPhone (6) to it. I surprised myself last month when it ran out earlier than expected. I assumed I'd jusy done something silly!. Today with my new month's data allowance I thought I'd better take a look. With the ' Data Plan ' app on the iPad I can see that I am racking up the MBs . When I disconnect the iPhone it stops so it's not my macbook or the ipad. Reconnect the iPhone and off it goes again. The phone isn't using its own data either it is just when connected to wifi. If it was an unlimited wifi that would be fine I guess. It's up to 120MB and I don't want to just leave it and see if it stops. My software is up to date 8.2 I've turned Automatic Downloads off. Photo streaming and all that is off. Any ideas any one? Thanks