Our Cannon Gas oven developed a problem yesterday. When we turn the switch up, there is no change in the height of the flames. The oven will ignite ok, but remains on what appears to be a low gas mark whatever we turn the switch up to. I have removed the jet and made sure it was not blocked at all, but this made no difference. I turned on (and up) the gas with the jet off, and I could hear no difference in the sound of the gas coming through. Does anybody have any ideas of what I could try next please. Any help much appreciated. Thanks