Avanta plus 24?

Not specific to you, as I do not know the model.
General things to check. From what you say, you get hot water, but when heating is called for fault.
check pressure, usually about 1 bar when cold. try turning the heating control to a lower setting, it could be over heat sensor, do you have thermostatic radiator valves, can you hear the pump running, If no pump, it would overheat very quick and close down, open the unit up, on end of pump, a big plastic screw, if you remove that, some water will come out, but you can put a screwdriver in to turn the pump to check if free. if stuck new pump. hope this helps a bit

SD
December 2011