Out of interest, how did you pair this with the C-8? I had the exact same problem you originally described, namely the same UK strip plug, with the same manufacturer and the same application data in the device details, not working on the C-7 hub.
It's very useful to me to have a strip plug with individually controllable sockets, so I bought a C-8 hub specifically to try to get this working, but I can't even pair the plug now.
I migrated my C-7 to C-8 using the cloud migration tool, which appears to have worked very well. However, the switch still didn't work in the C-8 straight away, so I removed it with a view to pairing it again.
I am now totally unable to add it as a new device and no amount of switching it off for a while/holding down the switch for 5+ seconds is resolving the issue. There doesn't appear to be a way of factory resetting it as far as I can tell?
I see you're saying it now doesn't work again as of 2.3.5.124, but surely I should be able to add the device at the very least? I'm on 2.3.5.125.