I think I have finally worked out the archaic piping in this flat, but
have a question for anyon knowledgeable out there... There are two
cold water pipes. One is large and iron and feeds my cistern, and the
rest of the building. the other is small (15mm) and copper, and makes
an awful racket from time to time.

It seems to me that the large pipe is probably low pressure out, and
the narrow high pressure in. Obviously with a high pressure pipe there
is lots of scope for noise. This noise is oftem of varying
amplitude, and is sometimes accompanied by a whistling. I would guess
that this is due to the ballcock causing pressure fluctuations within
the pipe, and also acting like a musical flute. Does this sound
plausible?

Is the best bet to replace the cistern valve? I would guess that
vibration and noise is most likely as the pressure in the pipe is
varying due to float movements. A new valve that switches from full
flow to zero flow rapidly might be better...?