I checked the pressure switch contacts with a meter. There are 9 wire
connections (labelled 11,12,13; 21,22,23; 31,32,33) on the pressure
switch
which seems to indicate 3 switches (each with 3 contacts). They are
wired
such that all the three switch centre poles (12,22,32) are wired together
in
parallel, all the NC (normally closed) poles (11,21,31) are wired
separately
and all the NO poles (13,23,33) are wired together in parallel. eg:
11-
-12
13-
In the quiescent state 12-11 is NC, 12-13 is NO.
When I blow & hear the 'click' for that switch, 12-11 opens but 12-13
does
not close?
It's exactly the same for the other two switches? Could this indicate a
faulty pressure switch but if so why would all three NO contacts fail?

I also check for blockages as you suggested but it looks fine.

The three switches should, iirc, all come on at different pressures
but should otherwise behave in the same way.

They do come on at different pressures and they do behave in the same way.
What I'm a bit confused about now is why each of the three switches have a
third contact (which is wired) but always remains normally open?