Overcharging for data used.

The internet speed and quality is very good but....... My wife and I are staying alone and we have the broadband internet with 30gb anytime and 30gb nighttime package. NEVER in my life have i encountered data being used so quickly as with Supersonic. We only watch youtube, Netflix and Catchup. Last night we watched Youtube xfactor auditions for 1hour and my supersonic statement shows 6.78 gigs used in 1hour. This kind of thing happens EVERY month and even after reporting it they say they can't say which device is using all the data. We know that only 2 cellphones and our tv are connected. It is simply impossible to use 6.7gb in 1 hour! We used 86gb in one month according to them. After the first incedent we changed all passwords but the problem stays the same. I am 100% sure the overcharge for data being used. I don't know how to prove it or what to do.