In article .com,
"Guttorm Christensen" writes:
There is only one pipe for the water supply into the flat. This is the
'mains cold water supply'.
The pressure regulating valve is the third unit on the pipe coming into
the flat, and previous plumber identified this valve (whatever it is
called ;-) as where the pressure is reduced.
The valve itself looks very similar to URL:
http://www.plumbworld.co.uk/343-1890, the only difference being that
there is no nob to change the pressure.

I wonder if you are in an area (London only AFAIK) where the
water pressure has been deliberately dropped to reduce leaks
from the water mains? Ask your neighbours if they have had
a similar experience over the same period. In that case, you
might just need to remove it, but that might not leave your
installation safe if the pressure should rise again.

--
Andrew Gabriel