Unaccountably high usage

[#1]

For some weeks now we've been bewildered by our high usage (including some strangely high overnight activity).
I've just started monitoring the Belkin router: so far nothing strange. I've also installed Net Meter on our pc's; over the last 48 hours or so this is showing much less than Plusnet's figures - the 5.95 GB that we are supposed to have used on Tues. 9/10 is nowhere to be seen in our own logs.
I'm not sure what to do next. Any advice? Thanks

[#2]

Do you connect to your router via wi-fi ?
Is it password protected ?
Is one of your children, or someone else in your household using a Smartphone ( via wi-fi ) at night ?
SKY+HD box downloading films / programmes on an auto download ?
Other PVR box downloading films / programmes on an auto download ?

Try turning your router off overnight a couple of times.
Do you still see high usage ?

[#3]

Switching off the modem/router as PlusComUK has suggested is a good idea if there is no other explanation as that will show whether something unusual is happening. One of the Plusnet team should be able to look at this recent usage and give you a breakdown of what it is.

[#4]

Thanks
Wifi,yes, should have made that clear. Two smartphones plus iPod. Having said which, we have not been able to detect any strange usage by these. Wifi is activated on Nokia and HTC phones briefly to look at emails when pc not on and no phone signal; but we are unable to detect anything else. Likewise the iPod sometimes connected but virtually no activity. Yesterday we are supposed to have used a whopping 5.95 gb. whereas the net meter shows approx 700 megs on pc's! Similar discrepancies on other days recently.
Will try turning off at night, although of course daytime problem remains, However, any more detailed breakdown from Plusnet team would be welcome.

[#5]

I'm creating a ticket on your account with the hourly breakdown of yesterday's usage so you can see what times the most data was transferred.

[#6]

Thanks Chris. I look forward to seeing that. We haven't been able to understand our high usage (allegedly Streaming) for some time now.

[#7]

You should be able to see it at https://portal.plus.net/my.html?action=questions&?helpheader=questions

Following on from a question asked earlier, is your wifi secured?

[#8]

Sorry missed the question about security. No we weren't secured - thought that living in very rural area we were safe from piggybacking. Anyway I've now WPA/PSKed the router.
Thanks for the timeline.
Still bemused by amounts: Skype for an hour (no video); 20 mins streaming; and quite a lot of surfing without download producing 6 gb. off allowance. It's odd also that the usage breakdown shows that the vast majority is 'streaming', yet we currently do so little of this for fear of exceeding allowance.
Will turn off router at night for a while. Also. would like to monitor iPod more closely. What's the best way of checking if it's using lots of bandwidth?

[#9]

HD streaming uses up a very large amount of bandwidth/usage.