How early to arrive at The Scoop in London for the free films?

Hello, I will be in London in early September, and am planning on attending a free film at The Scoop as part of the More London Free Film Festival. I know we have to arrive early as it is first come, first served, but how early exactly should we arrive? The film starts at 7:30 - would a 5:30 arrival be sufficient? Thanks for any help you can give me!