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.