Thanks again.

Apparently the reason for replacing a part rather than cleaning it was
down to the legality of having to dispose of the chemicals afterwards
in the right way.

I just looked at the pipes the chap said to cut and place the pump on -
the mains water inlet and the HW outlet. Would that still have no
effect as you mention above?

To be honest I'm desperate to perform what you did and remove the part
but I admit my plumbing is basic. Is it a case of 'unscrew this, remove
part, unclip that' etc. or were there situations where specialist
knowledge/tools or experience is needed? The engineer mentioned the
tricky bit is normally putting the heat exchanger back in.

Neil