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Turn off HW at programmer or turn cylinder stat right down so system is driven by room stat only. Turn up room stat, is demand reaching boiler? If not problem is with motorised valve. Need to know if orange wire becomes live.

You have bled the rads but have you bled the primary (hot water) circuit? Is there a coil upstand bleed nipple near the cylinder?

But is there demand from the room stat? Turn it up high and feel the pipe on the heating side of the motorised valve. Is it hot?

"Diverting through the HW tank stops the noise all together " Does the noise stop with HW on only; CH off? If so, as boiler and pump are running with no noise, problem must be in CH circuit. Resonance and/or partial blockage in radiators or pipework.

Are the pumps on min. speed?

Chris, The boiler needs to run for longer. Is there a higher flow setting? If the return water temp is cold the boiler thermostat is cutting the boiler too soon. Do you have the boiler manual? Can you disable the thermostat by linking across to force the boiler on longer?

If the boiler thermostat is shutting down the boiler after 3 mins but the cylinder stat and/or room stat is still calling there is insufficient circulation. Heat is not being drawn away from the boiler. You have bled the hot water circuit upstand but have not mentioned bleeding the...