I had checked the deleted folder and cleared it out to no effect but::
I rang the Cloud Helpdesk and I have to eat humble pie here. I was utterly convinced what they were telling me was incorrect. I had backed up Outlook files (*.pst files) and I was told that this was what was causing the problem. I was unconvinced since the size of the file I was backing up could not possibly account for the difference. I deleted the files out of the BT Cloud folder and hey presto, I am now using 4.1Gb.
Thank you BT Cloud Helpdesk and apologies for my irritable manner.