What to pay attention when putting flooring in the attic
Do I need to cover the insulation?
If there's a vapor barrier installed it'll be on the warm side. You can simply install flooring with nothing else over the insulation. I'd probably install a vertical board near the edges of the space to prevent items from falling off the floor and to prevent contact with the insulation. 

Are screws appropriate?
Sure. Whatever is convenient. As this isn't a living space, squeaks from foot traffic aren't a real concern. 

Do I need to leave any space between the timber pieces in case they expand?
I assume you're referring to the floor joists. No. What you may need to allow is a gap between your flooring boards, depending on what you use. Extreme heat in the summer could cause buckling. Follow manufacturer recommendations.