OK, now I know it's not the TV, as the Mux 1 signal came back on this evening (after dark) for a while, but was cliff edge stuff and occasionally dropped out altogether. So now I know that it's either a transmission or an aerial or other reception problem.
Also, I did notice that on a big Horse Chestnut tree directly in the line of sight between the aerial and the SC transmitter the leaves have all changed colour - maybe this has increased the attenuation to the point where all Mux 1 signals are borderline or blocked, but somehow allow other frequencies to pass ?
FWIW, my problems closely replicate those detailed at the URL given by fizban_UK.