what causes the igniter on cannon oakley oven to start clicking when alrea ?

i have recently bought a secondhand cannon oakley cooker,model no-(10515g mk2) using push switch to ignite (ring's and grill) no problem, when using the oven it light's no problem but after a period of approx 10 minutes the igniter starts to click constantly by it's self. i suspect it could be a faulty micro switch oven knob, could there be another cause for this fault. if so what is it.