I've got a New World Royale 600 gas cooker with a self-lighting oven - you
turn the gas on and the sparker clicks away until the oven lights.

First, how's it do that then?

Hardly rocket science.

Second, it only works when it feels like it - about 7 times in 10. Any
idea
why?

Flame sensor, igniter, knob position sensor or any cables or PCBs linking
them, I'd think.

Christian.