Silent Witness Season 1 Episode 103 Darkness Visible

When a death occurs in police custody, the footprints of the man who was sharing his cell are found all over the body. It looks like an open and shut case for Dr. Sam Ryan. But just as the investigation appears to be over, Sam receives a note written on police notepaper that re-opens the case. It seems that the real culprit may have been a police officer, but which one?