Tricity Bendix gas hob oven?

I have a gas hob and when I remove the black disc and then the circular metal part, to enable you to clean it, I have noticed that one of the plastic gas pieces has broken off. this means that if I try to ignight this particular hob, the gas does not come out of it in a controlled way. I want to replace this part but I do not know what this part is called or even if it is possible to do - gas is not something I would risk myself and would happily get an engineer in to sort!