How can I fix the gas oven that suddenly won't work?

The gas oven on our New World Series 2 cooker has stopped working. Gas supply to the 4 gas burners is fine and they light OK. Electricity supply still works fine - the timer light and auto-ignition for the gas hobs still work fine. But the gas oven control dial fails to cause the gas to flow to the oven and the oven auto-ignition does not function either.
Any ideas what I do to fix it? Cooker is probably over 8 years old.