Wind comes through the back porch (door, roof, walls, window — project for another day), makes its way through the back-door, and blows open the kitchen door. Time for a little draught-proofing.
Brushes on the base of the door and seal round the outside of the 3 other edges should do it.
Nope, also found wind blowing behind the catches on the door frame… so a little brown frame sealant on the outside is also required. Job’s a good’un.