Main oven will not ignite? Cannon Dorchester?

I have a Cannon Dorchester gas cooker with doubleoven and eye level grill. Whilst this may not be the newest in the range it is a perfectly good cooker. The problem is that the main oven will not ignite, no spark is emmited when the knob is turned. As it is not possible to light the oven any other way I would be grateful if you have any suggestions as to how to rectfiy the problem?