Well, there's a provisional installation. I just leaned out of the window on the left, where the lower pane opens, and put the box on the window sill of the other window, where the top pane opens. I feel pretty sure that the box is so heavy that the wind will never shift it, and that is east-facing so it will be sheltered from the worst of the gales in any case.
Those that claim to know say that you should put the box at least 5 metres up. That is 4 metres but worth a punt for next year. if the swifts ignore it I might consider getting it installed at soffit level, but I'm not confident that our bricks are strong enough to support the box's weight. This one is actually designed to be built into a wall.