Yes first step is to upgrade your ISP offering, unlimited ain't that dear anymore, heck you might get a sweet deal - faster access and less dosh to fork out (I did). Give your ISP a call and say that you want to bugger off as you have seen a sweeter deal elsewhere. Punch in your postcode on broadbandgenie or broadbandchoices and check of offers, might even get a £50 cashback
Of course check if there is "a naughty" playing around on one of your computers, easy enough to get install something. Heck it could be an update that is stuck. I had one on my tablet, I think it downloaded the newest update at least four times before I paid enough notice and searched for a fix. And as said up thread an Win update now easy clocks up Gb's. Mobiles especially likes to update every 1-2 hours, it does feels like it, and if you haven't told it to hold its horses, you have updated an app 2-3 times in a month, yes only 10-30Mb but it soon adds up.
Even an phone connected on your wifi - farcebooking, Instragraming and twitter might be light use for some, but boy do they like their bandwidth. (side note: why is that, that an app that is doing exactly the same and was about 10Mb is now about 200+ installed??!?!?!)
Before I got this new job, where I know my use has gone up, we chewed through 200+ Gb a month, let's post these photos to a farcebook gallery, view a few things on demand and that wasn't helped that I like to play around with various OS's in VirtualBox, so one download is quickly 1Gb before I updated said OS, heck a clean Win10 I think if I remember right just shy 5 GB.