CH works fine but when hot tap is opened water begins to heat up, but when 4 or 5 lights on panel are lit boiler shuts down and water goes cold. Then it comes back on and cycle repeats itself, all the time emitting a high pitched whistle (but only when CH is on). I have changed the overheat thermostat, removed and cleaned the diverter valve and changed the 3-way spring kit. None of this has solved the problem. Any ideas please?