The Great Conspiracy Theorist....

[#1]

In the main - I keep my iTunes Synch "Via Computer" only, rather than synching over the air.  I don't really need that level of connectivity. I've double checked what is synching and most of it is via direct connection.

[#2]

Quote from: jelv

For Windows PCs I suggest you install http://www.softperfect.com/products/networx/ to monitor usage (free download at the bottom the page).

Have you done this? It's usage report has a tab to show you how much is used per application.

[#3]

Ironically - I'll check these out after midnight - when it won't - ha ha! - affect my data!  
Edit: Seems like I have used Anti-Malware before - it tells me my "Free Trial" has expired!

[#4]

Uninstall Malwarebytes then reinstall the free version.
By default it tries to install the pro (or similar) version which has enhanced functionality for a while then expires. Make sure you tick the right box when installing

[#5]

Not sure that you have to uninstall after trial of Premium -I didn't when the trial period expired recently on my wife's XP netbook -just cancelled a few nagging messages and carried on using free version.

[#6]

Quote from: jelv

There's a well documented issue with iTunes gobbling huge amounts of data usage after a recent update. See https://discussions.apple.com/message/23710128 Unchecking Internet Explorer bookmark syncing in iCloud may solve your issue.

Having a suitable Software Firewall that controls the Outgoing can also be beneficial BUT you need to understand how to use it and customise it.

[#7]

*Shuffles in*
*Mumbles incoherently*
*Looks down at feet* 
I think I might have found it.
Going through Task Manager later on and looking at "Processes" I saw a programme operating that shouldn't have been.
I suspect that when I updated it last it pinned itself to the "Run at Start up" - so did so when we switched on yesterday - unbeknownst to all.
Always check the label folks!
Thanks for the suggestions - Networx is useful!

[#8]

Glad you've got to the bottom of it. Always worth checking what runs at start up, as well as what is running after booting has finished and it's supposedly idle.

[#9]

You're not the first and you won't be the last.
Don't forget to eat humble pie to your wife for accusing her "non Apple product" ...... 
Care to share what the offending program was (just for future reference) - don't worry if it is something you'd rather not share 
Hopefully that will be the end of your issues

[#10]

Crikey! Can't remember now. 
Turns out to be academic - as we all seem to be using more data now anyway.  Tried to keep a lid on it, but generally it's just a sign of the times.