7mm, triple shield RG6. But don't try and buy your own - VM know from bitter experience that the vast majority of customer bought cable looks like the right thing, but doesn't work like the right thing. Cable internet uses a much wider frequency range than other domestic coax uses, and even "satellite grade" or "guaranteed VM compatible" mean nothing, leading to noise ingress and poor service that can affect you and all neighbours on the same segment.
Sounds to me like you might need to have a £25 "non fault" call out?