@GregJam 

Hi, welcome to the community.

You could try to check the system logs on the web UI-http://tplinkdeco.net.

Normally it would record as link down and link up. But it did not record what caused the link down.

Have you seen the main Deco S4 turn solid red during the disconnection?

It is also suggested to unplug the main Deco when the internet dropped out and connect a computer to the Virgin modem, then check whether the computer could still get an internet connection.

And There hasn't been a new firmware update to Deco S4 and I don't think it might be related to firmware updates.

If possible, I think you could revert the Virgin Media router back to modem router mode but disable Virgin Wi-Fi; then set Deco S4 in Access Point mode to provide the Mesh Network around the house:

How to set up Deco to work in Access Point mode

Thank you very much.

Best regards.