Roger Mills wrote:

So that's it in a nut shell, as I say I've over simplified for
brevity, but that's why you should use RGB rather than CVBS :-)

Many thanks for the detailed explanation. I think that, encrypted in there
somewhere, is an assertion that feeding RGB rather than CVBS from the STB to
the TV results in a better picture because the signal undergoes slightly
less conversion processes along the way, even though it isn't transmitted
digitally as RGB. Is that a fair summary?

Spot on, using the RGB output, significantly reduces the amount of
processing, and therefore impairment that the signal will suffer.