When turning on the oven the self-ignition starts and continues firing long after the oven has lit, it continues firing intermittently for the duration of the cook/Bake, which may be 3.5-to4 hrs.
At other times the ignition will not fire at all when trying to light the grill/hob! (Using Push Button)
I have checked the push button for continuity and it is ok.
The Ignition worked before the clock had been set after turning the cooker back on.

Bob F13, February 2009

Most ovens rely on the spark electrode doing 2 things firstly lighting the oven but then causing flame rectification so once a flame is noticed it should stop sparking.It could be a dirty contact,a broken spark probe or sparklead,or a faulty Pcb if you have one.Not sparking at all could be a faulty ignition switch or a spark generator fault,though this is unlikely as you say it works more than not.

Tony S, March 2009