It's a bizarre feature - two of the windows in the flat are theoretically openable, except there's no handle fitted and they're secured shut. Or at least I thought they were - I've noticed that one shifts in the wind but never enough to actually open before.
Anyway, I went spelunking through the bag o' tools and found a suction cup to use to pull the window shut, pried a blanking plate off to reveal the slot that the handle would turn if there was one, and improvised my own handle with a screwdriver. That window is now definitely shut, and I can glue the blanking plate back on later.