How can I repair my ignition for a gas hob?

I have a smeg A1-5 oven, but one of the hob knobs has stopped creating a spark. All of the other 5 knobs work perfectly. I can still use the offending hob/knob by depressing a different hob knob to create the spark (all 6 should spark together, from any knob) but this is very annoying. Please can someone help? Do I need to replace the knob itself, and if so, where can I find a manual telling me how to do this? Thanks.