help help help?

Please someone help we have had about 4 heating techs out all safe gas registered and no one can help its cost us so far in the region of £400+ and still not workin ...we have a combi boiler when you put heating and hot water to auto on timer it fires up and radiators start to heat once you turn the hot water tap on it starts were its tryin to firer up all the time then just cuts out and overheat switch comes on.if we put it to hot water only is struggles like its firing up all the time. If we turn the timer to off then we get hot water no heating. we have been told its the main board which we had replaced then we have been told its the fan modulatin board PCB which we have replaced several times as we were told the new one we bought might have blew or be faulty....someone HELP.