It would make sense for Bamber to leave the bathroom window ajar when he went for supper. To ensure a quiet entrance later. Nothing to lose in doing this.
Nevill, June & Sheila may not have entered that room before going to bed. If they did go into that room, they may not have noticed the window was ajar. It was summer so the room would not be cold.
If in the unlikely event, the ajar bathroom window had been shut by Nevill, June or Sheila, that was not a problem for Bamber. He knew how to get into the closed window.