Hello, I have a Vaillant Geyser Mag 250/6 instant water heater. The pilot light went out. I can light it (using a gas lighter) however the burners do not work when I put the hot water on. When I put the knob to the first position ‘*’ the light stays on. I then move the know to ‘I’ whilst I hold it fire stays on, let go and the fire goes out. When I move it to the last position the knob moves out and the pilot goes out. So the pilot light only stays lit at the ‘*’. Any ideas?