Help - I've gone over my usage allowance!?

[#1]

I've only been with Plusnet for 2 weeks and I've gone over my 1GB monthly usage allowance.
I've never exceeded an allowance with previous providers and had broadband for 5 years or so.
My allowance usage chart is showing 70-100 megs per day and I can't understand why. I only use the computer to surf the web, my average usage is as follows:
Surfing  2 hours per day (ish)
Email  5 per day (no attachments)
No other downloads music/videos etc
No on line gaming
I do leave my router on all day/night but the computer is turned off apart from when I'm surfing.
Can anyone help?
Many thanks
Simon

[#2]

Just gone wireless by any chance?
Maybe someone's piggybacking?

[#3]

No - I've been wireless for about 3 years so I don't think it is that, although I haven't got  a clue so it could be!!!!!!
I do have a WEP(?) password on my router would this stop them? 

[#4]

Have you been looking at any streaming content?
audio, podcasts and most especially video?

[#5]

Yes I was going to say the same .. that kind of daily usage could easily be consumed by surfing, depending on what you surf.
Things like YouTube and even Facebook can have embedded videos which can take up the usage now. It's also possible that a site might have video adverts for example. (I seem to remember on here a while back, someone had something similar and it was narrowed down to some video advert).