They’ve somehow climbed on the roof of the stand I presume and then either used the skylight to get onto the gantry where the flags are and down that way (not sure how they would get back up minus the ladder) or they’ve had access to the ground and a ladder? Climed over the gates in the corner perhaps and thrown a ladder over?
Fair play to them either way. That gantry is high enough for me let alone the f*cking roof.