Like me it's knocking on a bit. This same pump has worked flawlessly for 31 years so it is time for a new one.What I did, for a long while until I sussed out the problem with the over-run stat, was to simply turn the stat in the hall down over night. It had been like it for a long time before Wifey and I got together, in the house she and her ex had owned. A few heating engineers had tried to solve it but not done so.
Which is weird as I know zip about CH. But I somehow managed to find it and swap it. Problem solved for a while. It goes every now and then as does the thermo couple, so I keep spares and can now fix it quickly.
But a sluggish pump is another matter.
est of luck getting it swapped out.
Any ideas why it has gone fubar? I thort you had been very thorough with keeping your CH fluid clean and full of protection stuff.