Fixed the problem, it wasn't an issue with the network at all, but a recent update to chrome & edge. If you google "Google Chrome's new post-quantum cryptography may break TLS connections" you'll...

@David-TP I've already tried changing the DNS as you descibe, but with no change to the behaviour. I'll have to wait until no-one else is using the network before I can reconfigure the virgin modem...

Very strange problem that has me tearing my hair out. I've got a mesh network consisting of 3 XE75 nodes, connected by an ethernet backbone and accessing internet via a Virgin modem (I'm in the UK)....