Zenith 55 Cooker - Oven not working?

Everything in my cooker is working to perfection. The only problem is that now my Oven has stopped working. The timer (clock) is working fine showing the correct time and it is not on auto.

What seems to be happening is I turn the oven on, the ignition happens and the oven turns on the fire, however it is not as powerful as it was a day ago. Turning the flame up all the way to gas mark 9 and it looks like I am cooking with a candle.

Any ideas?

Much appreciated.