Problems with ignition system on Rangemaster 55 gas cooker?

The 'ignition system' ie the push button arrangement on my Rangemaster 55 single oven gas cooker, no longer ignites the gas when it is pushed and I have resorted to an independent battery operated igniter.
I am uncertain about whether this is something I can easily repair/replace and would appreciate your guidance. Also, if the 'problem' I am describing is obvious to an expert and I need to purchase a replacement part, what do I need, can you supply it and how much will it cost?