how can i fix my belling fusion 136 electric fan oven?

when i switch the main (fan) oven on, the fan starts but the element does not heat. i noticed the red neon indicator light(on the panel,below the switch) does not light either
i got a repairman to look at it, and he said it was the thermostat .He never returned to repair the cooker, which prompted me to investigate the problem myself. I have since discovered that the thermostat is working perfectly. Further investigation led me to the switch itself.When i tapped the switch with a screwdriver , the element and neon light worked, problem solved. I thought. Now, no matter if i tap on the switch or not the element nor neon will work. I took apart the switch, thinking it was a loose or bad connection or even a dry joint. I couldnt see any problems until i took apart the little black box at the back of the switch. This revealed sooty deposits around the contacts, and the contacts to be dirty ...............what do i do next??