I have a V6 and a TiVo connected via Powerline plugs and I can see recordings on each box from the other when looking at My Shows. At least I can until I add another Powerline plug for my Raspberry Pi Server. Then I can no longer see recordings on the other box and I get a C423 error. (There may be a problem with your home network or the network connection on your TiVo boxes, or the TiVo box may have been unplugged)
I can still connect to both boxes with the Virginmedia TV control Android app but the boxes cannot connect to each other.
I think it is only an issue when the Raspberry Pi is connected. Previously I was using the same Powerline adaptor to connect a Samsung TV with no problems but there was very little date sent over that connection. I have not tried connecting anything else as a test. All the devices connected to the Powerline adaptors can be seen on the network. If it is relevant the Powerline adaptors connected to the V6 and TiVo are a pair I bought a few years ago and are 200MB/s and the third adaptor is a Netgear PL1000 provided by Virginmedia.
As long as the Powerline is active before the Pi tries to connect any Powerline adapter should work really.
It could possibly be a compatibility issue between the different Powerlines. Is it possibly to connect the Pi directly to the hub - or an alternative set of Powerlines - to see if the problem persisits?
Since I had a spare PL1000 I connected the Pi to a separate Powerline network consisting of the two PL1000s and everything works. That is how I have left it now but it is annoying that it doesn't work with only three Powerline adaptors on the same network. It wastes another socket since the PL1000 isn't pass through.
I have an update about my issue where a TiVo box and a V6 box were unable to share recorded programs when using powerplugs if a Raspberry Pi was connected to a third powerplug.
Previously I had my TiVo box connected via two old intellon based powerplugs and the V6 connected directly to the router. This worked until I connected a third netgear powerplug for my Pi when the communication between the Virgin boxes was broken.
One of my old powerplugs stopped working recently so as I already had my Pi connected via two netgear powerplugs I tried adding my remaining old powerplug to the same network as the two netgear powerplugs and connecting my TiVo to it. I was not expecting it to work as this should have been the same as my original powerplug network but I was surprised to find it does work.
So two netgear powerplugs plus an intellon powerplug works but two intellon plus a netgear doesn't work. I have no idea why just pleased that it does.
Sorry to hear that you haven't solved the issue that you're having slartibartfast1. I have done some background checks on the system and everything is working as it should from the connection side of things. As my colleague mentioned earlier it maybe a limitation with the powerline adaptors when another device is added to the network.
Unfortunately we can only advise that the connection is working from our end and there seems to be no issues across the network.