The message
from "Caroline" contains these words:

I once spoke to anurseryman about it. He said that the moss was an
indication of a poor soil. . For luck, I tried to put on more
fertiliser than I usually do, as the nurseryman advised.

He was ripping off the OP.

Result: A
truly bumper crop of moss on all the beds.

I have lots of moss in the garden (NW and shady during the first half of the
day), I never used any fertiliser and my plants are growing really well. Is
moss truly a sign of a poor soil?

No, it's a sign of acid moist soil. Moss grows perfectly well on acid
moist highly fertile soil, as the OP found out.

Janet.