Hi & many thanks to you both for taking the time to get in touch.
I can tell you that VM recently updated the app & after several reboots of my tablet it has now started working! I don't really know why as I didn't change any of the settings, but I'm not arguing with the result. My tablet runs on android v6.0.1 which until recently I understood wasn't supported, so things are apparently changing.
Thank you for your messages, it's good to see you all here. As StopItRawr has mentioned, the reboot of the device usually resolves this sort of fault. Can you let me know if you're suffering any other issues or if this problem has remained?