For the last few months we’ve had a strange infestation. Sometimes every day, sometimes once a week, a small tortoiseshell butterfly appears out of nowhere in the living room. It’s always when the heater’s on and the butterfly basks in front of it, throws itself at it frantically, then flaps around the room until I can eventually guide it out the window. Then the next day… another appears. Where do they come from? Do they think my heater’s Spring?