My Britony II water heater refuses to light?

The pilot light will not light - even when I remove the case and apply a lit match (which is the way I've had to do it in the past whenever it goes out, even though the piezo ignition is sparking).

Also, unlike previous occasions I can't even hear the faint hiss of the gas coming through the pilot when I turn the knob ready to ignite. Clearly no gas is coming through (I've checked the supply and it is on).

Is there a valve/control that could be at fault? Thank you.