Really do check your antenna connections even the wall connector if you have one. Make sure all fittings feel tight as the standard Belling & Lee coax connectors are rubbish imho.
Turns out I had a slight crack in the solder of the wall plate connection internally.
Whilst I had really good signal strength and quality on all muxes apart from one, the signal quality of the one would vary from 0% to 80% randomly even though it showed 80% signal strength. After swapping out all cables it was still randomly changing and making no sense as I thought if the other muxes had good signal it can't be a cable issue as surely they would be on or off. Replaced the wall plate socket with a standard plug on the end of the cable coming in to the house to test and boom. The dodgy mux had 100% quality and has had ever since.
So whilst I really don't understand why a slight dodgy connection would only affect the quality of one mux it seems to be the case.
Hope that helps.
I bought an attenuator from Amazon as I was having HD channel signal issues...only with the latest Youview box mind you. Haven't had an issue since.
My problem was that the incoming signal was too good. An attenuator did the trick. 👍