Credit where credit is due, my problem now seems to be resolved: after months of frequent wifi dropouts, it's now fixed. I don't know what Virgin did (and it was certainly a 4-hour nightmare trying to talk to them). But they must have done something, because now we have continuous wifi throughout the day. That's confirmed by Broadband Quality Monitor (something weird happens around 2AM, it was big red spikes the previous night, but that's hardly a problem)...
On the wider issue: yes I get that wifi transmission throughout the house is complex and can't be considered Virgin's responsibility... it's the customer's responsibility to position the hub correctly, buy boosters if required, etc etc. BUT I don't think Virgin's responsibility ends with signal entering house: there should also be a good and continuous signal emitted from the hub, and if that's not happening (as I think was my case) then I consider it to be Virgin's responsibility. But anyway, in this case they've somehow fixed the issue, so thank you Virgin, appreciated.
Thanks for coming back to us, I have had a look our end and cannot see any issues at all, I have run diagnostics also and nothing has been picked up, it has advised for you to try try a pin reset on the Router just to rule out a software issue, first check all the internal connections are hand tight then push a pin/ paper clip in the back of the Router where it says reset, push in until you feel it press then hold in for a timed 1 Minute, do not reboot during or after and allow 15 Minutes to settle afterwards?
Thank you for your reply Paul. Yes, I've done exactly what you describe. Then it works, for a while. But sometime in the next 24 hours it'll likely go down again. As it stands, I have to pin reboot every day.
I can see before I advised to do this the Router hadn't been rebooted or pin reset for at least 7 days so maybe it wasn't pressed in for the full 1 Minute as advised, now you have done this please run the BQM for the next 72 Hours.