Are you using wireless or Ethernet connection? If you are using wireless - maybe one of your neighbours is using your bandwidth to download stuff. If you are using wireless make sure it is using encryption to stop others stealing.
Just a thought
That has already crossed my mind. I am encrypted but I changed wireless key and Admin password just in case. Even unsubscribed from BT-Fon, for good measure.
I had a look at the 2 tablets I have via wireless and checked their usage log for the past month and they are both minimal.