If neither device will download I can only see two places it can be stopped. If your using the same Lan for all devices, then there can be a firewall block on a port in your router. Otherwise and my guess is that Virgin.net blocked it. Check with them.

Since Thunderbird does not run on phones or tablets I would say your problem is not with Thunderbird. Contact your email provider.

kiwikate said

yesterday switched on pc as normal and no virgin.net messages would download. Thought maybe that large file on server might be blocking download but checked via webmail and deleted one possible large photo. But no change. Not downloading to tablet or phone either. Can send emails from pc. Have looked at settings and all seems well there. any suggestions warmly welcomed..

Thank you both for helpful response. Virgin media have corrected fault.