Thank you for this. I looked but it was difficult to tell as if never reset the counters before so was showing stats from way back. I’ve now reset the counters so perhaps over the next day or 2 some lift may be shed on this!
Please check your mum's mobile to see what apps she has and the individual settings for each app to see what is using the data. Also make sure that she is able to connect to her normal WiFi networks such as her home network.
Thanks Lisa , nothing at all has changed in her apps or usage, she has had this handset for 6 months and has been on the same VM tariff since 2011. I have set her phones billing cycle so that I can monitor app usage now. Chrome has taken quite a bit of data though
I got her to turn her data off to see if her allowance was still dropping and so that she would notice when her WiFi dropped. But as yet she hasn't received a 100% usage text, and her WiFi is stable.