Complex sentence: The late Sir David Bowes-Lyon, while taking a late stroll on the lawn after dinner, reportedly saw a girl gripping the bars of a castle window and staring distractedly into the night.
Split into simpler sentences: The late Sir David Bowes-Lyon supposedly saw a young woman while taking a late stroll on the lawn after dinner. Reportedly, he saw a girl gripping the bars of a window of the castle, staring distractedly out into the night.