Your is a sad tale so you probably won't appreciate being told you would
have done better with a small hammer and lots of gentle percussion on the
corner of one of the accessible flats of the union nut. With a modicum of
practice this becomes second nature to folks who do this job regularly.


Noted for future reference, along with other's suggestion of heat.



Finally, undid both compression nuts (top and bottom Ball Valve - pump's
aligned vertically) and hack-sawed through one of the compression
fittings
to allow all components to slide off the existing, non-moveable pipework.

That's when the real fun started...

Having now had to drain the system to get the original ball valves off
the
pipework / pump, and replaced with the screwfix ones I try to mate the
pump,
and tighten Big Nuts. Nuts (or ********, if you prefer) - would they
thread
onto the pump?, have a guess. Had to remove one of the Ball Valves to
prove
to myself that the threads were compatible - they were. The problem seems
to
be that the Big Nut will only sit properly, and flush, once it is fully
in
place past the valve actuator on the Ball-Valve, that is, in its final
position. Problem - if there's (for example) a pump already in place
between
two of these Ball Valves, tight fit and all with non-malleable pipework,
then you cannot easily (more later) offer the Big Nut upto the thread on
the
pump housing without risking cross-threading the nut.

Looking at the mangled remains of the old pump it appears the original
installation suffered much of the same problems (with apparent
cross-threading on one of the Ball Valves).

Two comments here. First ball valves are Crap with a capital C. After a
few years in position the ball corrodes such that when closed off then
reopened they almost always leak around the spindle.
I use gate valve type unions as they have a nippable gland and so avoid
this situation. OK sometimes they weep by when closed but its controllable
if you work quickly

Original ball valves were weeping a little, or were after I tried shutting
them off. Rest of system controlled by gate valves, although even with all
"off" I still got a feed somehow from the cistern, guess one didn't close

The particular pair you got must be a bit unusual as the problem you
describe here is not common in my experience.
(Gets worse doesn't it?)

Ah, that answers my question... I was about to hit the rush-hour and speed
over to Wickes for a replacement pair in the vane hope that they'd work
better when I finally got the 2nd on.



SO

The question is... for someone with experience of plumbing and central
heating... Just what is the trick of the trade to mating the Ball Valves
to
the pump bodies?, I imagine with play in the pipework it would be fairly
straight-forward, but for existing installations, with no such play, how
do
the Pro's handle this?


Install with a smoothly sliding fit between the faces of the unions:-)

Bah!, where's the fun in that

Regards