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The problem sounds like a blocked injector, this is often caused by dirt getting into the injector which sits under the burner.
With the burner cool to touch, remove the metal burner cap (A), metal burner head (B) making sure you dont damage the Electrode and thermocouple (E).
This should now have exposed the injector which is often brass coloured, nut with a small 1-2mm hole in the centre. Place a paper clip or some other stiff wire 1-2cm inside the injector hole and wiggle it around, turn the gas knob on and listen to see if you can hear gas hissing or smell gas. If you dont try again with the wire and tap the injector with your finger to help free.
Once the gas is flowing through the injector, put the burner back together (replacing the metal head - B, ensuring you align up notch C to the electrode and thermocouple, and placing the cap on top), turn on the gas again and check the burner ignights, and there is a good flame all the way around the burner.
If you can not clear the injector, you need to organise a Gas Safe engineer to attend, who can then remove the injector give it a clean and potentially replace it if required.