I think I may know the answer and I feel extremely annoyed that the settings are set up in such a way when you buy the phone in the first place.
So after speaking to Samsung there are 2 things you need to do.
1. Go to Settings - Data Usage - settings handset button - tick Restrict Background Data
2. Play Store - Settings button - Settings - tick Update Over Wifi Only and turn off Auto-Updates.
The reason you may be using a lot of background data on your android is because apps like Facebook and Emails are updated regularly and using a lot of data which you are completely unaware of whilst you are in Data Network Mode.
You can also control the amount of data that you use by moving the bar on the data usage graph to your limit or slightly below and ask it to give you a warning when you are close to your limit.
Hope that helps.
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