Your Von Hotpoint gas cooker is producing a luminous flame (yellow flame) because there is no enough oxygen for complete combustion. Make sure that your Bunsen burner is clean and the small oven air holes under the plate are not closed.
Almost all Gas cookers Ariston, LG, Armco, Bruhm, Mika, etc produce the yellow flame when combustion is not complete.
Also ensure that you do not have a lit Jiko in the same place as the Gas Cooker or there is enough air circulation in the room.