This. I don't know if you can get it in Great Britain (guessing) but they make clear duct tape - if not, slap something similar over the crack and cover the window with a cheap blanket secured tightly with push pins around the frame.

Looks like a double pane, inert gas filled window, and if the outside isn't busted, in theory, shouldn't have lost that much R-value (insulation ability) - might be something else going on that it exposed.

(Edit: maybe not? Bad picture.)

Anyway, do the above and feel around for drafts coming through it or the surrounding trim.