Bob Wilson wrote:
Yet another ground to contend with is whatever may come with a cable TV
connection. (Sorry for my ignorance, I am not sure how to translate for
the other side of the Atlantic! In the US we typically have a 75 ohm
coax coming in for this purpose.) At another residence I had about 3
volts between that ground and the ground for AC power.

A modern cable TV demarcation point has galvanic isolation.
With a TV there usually is no problem because it usually has galvanic
isolation in its input circuit as well (some ferrite bead transformer
between the input and the tuner).

For radio tuners that may be different.