Thanks to all those that replied. Just to feedback my experience:

Flushed the system, which to be fair didn't seem that bad - a murky
greenish tinge to the water, but eventually ran clear. Replaced the pump
as a possible solution suggested by Roger Mills, now all works fine.

(also took the opportunity to replace the (awful) ball valves with gate
values)

Tomorrow, when I can find the inclination, it gets a Fernox de-sludge
treatment, and after that's complete it's getting its yearly dose of
inhibiter.

Thanks to all.