So that was "interesting". Mrs LT was trying to use her phone to look at the Odeon website to check some film times, but kept getting blocked by Cloudflare.
We spent a good 15 minutes checking our network, DNS setup, all sort of things. Then she remembered that her Pixel phone had updated fairly recently and we discovered that Google had automatically switched on their own VPN.
And Cloudflare, for some reason, had classified any requests from Google's UK VPN exit nodes as being used by bots or scammers.
Anyway long story short: Google VPN off, Proton VPN on, all is well again.