Could be the leads shorting, could be a faulty spark generator, could be a faulty ignition switch or could be that you have spilt liquid onto the igniter button that is causing the continuous ignition.
the reason your main oven won't work if you turn the power off to stop the clicking is that the main oven is controlled by an electric solenoid.
the most expensive item would be the spark generator box which is around �65, we normaly charge �50 for a repair which includes the initial visit to diagose the fault, fetching the part and then returning to fit it.
terence makes stripping the cooker sound easy and sometimes it is ut depending on what cooker it is sometimes to remove the spillage tray you have to take off the front panel, including all the knobs and timer buttons as sometimes all the panels link together.