Hi Dave, thank you for your reply. I've run the check status on both boxes (and also the broadband) and no issues there.
I hadn't tried switching the boxes around, so I did that and magically, both boxes can access the other again. UNTIL, I put them back in their original locations, and hey presto, not working again! Do you have any idea what that means??
I'm not sure if it's relevant or not, but there's also some issue with accessing one of the boxes in the TV Control app. Until I switched the boxes around, I was not able to access the recordings etc on the 'Living Room' box via the app, although I could happily connect to the Kitchen box. After switching the boxes around and back again, now I can access the Living Room box, but I can now not access the Kitchen box!
All other online services on both boxes work perfectly, including Netflix, Catchup, OnDemand etc, and broadband is generally unaffected. I don't actually use the WiFi on the Virgin Hub as it's useless, so it's turned off. I've got a Netgear Orbi mesh network set up, connected to the Hub via ethernet. Until today, the TIVO boxes were always connected via ethernet to the main Orbi, but when on to support today, he told me they needed to be connected to the Hub, so I've done that and both are online still. The previous setup worked perfectly for 2+ years and I suspect I'll have to go back to that as the TiVOs are now likely not discoverable to other devices as they'll be on a different network subset.