Oh sorry, I didn't notice they were the same, but I read the comment "If you don't check the Show WiFi usage box, there is no tab" and blindly assumed the second screen had no tab. I've noticed that I can drag the white vertical bars to change the reporting period. When I do this I find there's no data usage after 26th January, which is presumably when I swithced off mobile data. When I contacted my provider they said I had a lot of data usage in Janurary (although my mobile records only 7.42MB. I've updated the bigger data using apps to specify 'Restrict background data'. I had this set on my wife's Samsung Note 3 (the Nexus is so much better, and half the price) and since I did it her mobile has recorded zero data.
I'm sure my my balance has dropped over the last day or two even though the data usage was a few weeks back. So, I am going to log onto my account daily and monitor my account, it's tedious but needs to be done. I've asked my carrier to switch off mobile data. If I find unexpected costs I will contact them. Unfortunately they do not itemise costs on PAYG so it's difficult to montitor. I very stongly suspect they have a glitch in their accounting process. I will try and narrow down the next occurrence and then follow it up assertively. If there is a problem it could be affecting a very large number of people.
Thank-you for everyone who has offerred help. I can't see what more you can do. If I find out anymore I will post here, otherwise if it all goes well from now I will try and remember to note it in a few weeks time.