Tech support call.
Is it possible that Virgin Media could block the /mt/ directory of a blog with a "403 forbidden"? But only that directory and only of that blog. I know they're prone to net nanny filters but this seems strangely specific.
Any browser (Chrome, Firefox, Edge), direct access via Virgin Media cable does this. Both wired and wireless router connection. Same machine and browsers via BT Wifi, or via Tor is fine.
@jbond with modern "network surveillance/security" kit an ISP can block anything they choose, but this does seem unusual or even a misconfiguration. (VM/NTL have traditionally used transparent proxies to save bandwidth, not sure if that is still the case).
I may be working on a site tomorrow setting up a network where VM is the provider, if you DM me the address that is acting up maybe I can carry out some tests..