It's screwed to the top using slotted screws but they are quite rusty. I've tried my cordless driver but I think the corrosion is holding them too tight.
I have an impact driver, but I'm concerned this could snap the screws below the surface.
Heat will help - Get an old well fitting screwdriver end, red hot and hold it in the screw until you smell burning wood. Then try will a well fitting ordinary screwdriver.