Thank you very much MHC. Your comment is most reassuring.
Yes could be a camera/phone is now furiously backing up photos to OneDrive, DropBox, Picasa, flickr and you've forgotten about it.
During the questionable period, about 230 jpeg images were downloaded from a camera into Picasa. I reckoned these were, say, 5MB each, ie say 1.25GB. However, surely this did not involve the internet connection. Is not Picasa on the PC and the images stored on the PC, too?
My friend and BT believes that an Acronis back up during the period was a cause. However, this is a normal, regular procedure and does not include backing up to a "Cloud" server, only an external hard drive. Furthermore, the volume of back up data would have swamped the usage allowance very many times over, not just 6GB!
I guess it must have been something to do with the images, but 6GB increase? Very odd!