Strange zone valve and pump behaviour?

[dpravo]

So circuit testing all the various bits is where i would struggle @Madrab I know how to test somethings eg the cylinder stat, i did that yesterday with my multimeter to make sure it was off before wiring in the new stat, but all the other components i don't really know where to test. The water is pretty clean, tend to treat it every couple of years with aomething like x400 rhen drain, fill, drain fill and add an inhibtor at the end. To me it doesn't feel like an issue with the boiler ( an ideal classic gas @Johntheo5 ) but what do i know really. i have monitored the flow pipe temp ( with ir thermometer) right next to the boiler and the boiler definitely stops firing when that flow pipe hits roughly the documented temperature for the boiler stat - e.g the boiler is currently set to '2' which the manual says is 61 deg c, and when i monitor the pipework and it hits about 62 the boiler stops firing. Now, i know you said not to but i bought another replacement part, i swapped out the synchron on the hw zone valve as it had a cheap knockoff synchronOUS motor which was getting noisy, so its got a proper one in it now.
After turning eveything back on it all seems to work, rhe boiler fired up, the hot water cylinder got up to temp and then the boiler stopped, the pump stopped and the hw zone valve closed. right now the prgrammer is still 'on' and i'm gonna use the shower later and see if everything fires back into life and the water heats up again.

 

[dpravo]

Ok.. update - the general behaviour of the system seems to have fallen in line and is behaving ( did i just jinx it?). i used a load of hot water, then after some time, maybe 5 mins, the boiler fired up and reheated the cylinder. After a little while the pump switched off. Then to test it wasnt a fluke i used a load more hot water and everything fired up and it heated back up again. wonder if it was the synchron after all? or did everything get thrown out of whack becauze of the unsesonal cold snap?! you watch.. it'll probably be all messed up again tomorrow.