Cannon chesterfield gas cooker?

I inadvertantley let a pen roll off the glass hood of my cooker. I assumed it had fallen down the back but as i have now found out following the smell when the oven is on and further inspection i.e. Looking down the vent with a torch it went down the vent and seems to have landed on the top of the oven and melted. I have tried to remove the rear panel with no success. How do i get into the area of the vent so i can clear it up, the smell is awful.