Thank you Hyph.
I will review my usage because you're probably correct to say that 25GB a month for Netflix is not that much.
I wonder why O2 might worry about the tethering though, especially if I'm paying for what I use...over or within the contract? I'll read the contract to see if it mentions it.
It's not a Galaxy note 7 thankfully, and yes I will check it all works before cancelling BT.