Does the ET8500 only have one tuner?
Personally, I wouldn't use those face plates. A 4 gang box with separate f connections or even individual modules would be better. The looping could be done directly inside. F plugs would be far better than the way you have screwed them in. Your problem may even be the shared connection plate with router.
You have not explicitly mentioned switching off all receivers and testing one at a time, there is a weird fault with Sky LNBs when that only happens when more than one port is used. Swappibg the cables on LNB end is another test.
You could connect the 28.2 wire to the Vu, then use the Xtrend in client mode. You then avoid using the cabling to Xtrend apart from ethernet.
Edit:
Virgin cable is of a very good quality. They cant afford to install using poor quality cable and come back because of signal issue. As above, bypass the face plate and use f-plugs. If it is still the cable, more likely the kinks and quality of connections on face plates. You have two really bad kinks in one of your pictures.I have a feeling the cable shown dotted in red dose not allow transponders at higher frequencies.