Hi, you mentioned you change the remote thermostat, what make and model is it?Have you set the C/H on the controller to "always on"?It sounds like the controller isnt calling for heat, if you turn the thermostat down then a minute later up does the boiler start?Another item to look at is the C/H actuator, which lets the hot water from the boiler flow past the radiators, if this isnt working the boiler is heating a small loop and reaches critical temperature very quickly, you could try manually opening the actuator to let the water flow.The fault your describing doesnt sound like a thermal couple \ heat exchanger fault to me.
have changed thermostat and no change i think more than likely diverter valve as there is a clicking sound as if something is trying to engage heat seems to get as far as the pump and a little further have just cleaned pump and all seems in order ?
sadly diverter valve very scaled up no movement have cleaned to my best and sprayed with wd40 I'm going to pressurise the system and hope it holds water !!!! ?
fired up well sadely has fast drip from diverter think because i had trouble lining up pin on diverter ?
i can't get the central pin out but what about ptfe past or pumbers mate with a5 mill rubber washer on top ?