Just had a call from the guy with the problem, apparantley none of the rads are getting warm now, not even the upstairs ones as before, could this be the pump as opposed to sludge?
It does sound like the pump could be on its last legs. It might be worth dismantling and cleaning it just to see how mucky the system is, but I'd still be inclined to go for new one.
Took advice given on here and swapped pump, problem was still present, added some inhibitor and balanced the system, this sorted out most of the rads, only 2 remaind cold. I think the valves are knackered on these anyway.
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