Hi Mat.
Funny thing now!! I have almost the same thing since last Friday and I can see the main sources in link rooms with "snow"!!! I think in my case is a matter of the RF modulator.
Another thing you should check is all cabling in your master tv and if any cable has just gone lose.
By the way, it cam happen some times, unplug everything from power and then unplug the ml cables from the sockets as well. After that, plug firstly the ml sockets and then connect to the mains.
Other than that I cannot think of anything since my tech knowledge is very limited.
Please provide with results.
Best regards
Spiros