A TIGHTLY-BOLTED door, a spy-hole and a secret knock were not enough to keep the frail divorcee Renee McGowan safe in her top-floor flat. Mrs McGowan, 55, was divorced two years before her murder on July 23 1975 and, as she lived alone, had done all she could to protect herself from intruders. Yet someone still managed to get into her top-floor home at Evans Towers, off Manchester Road, and strangle the quiet, well-spoken woman. She was found partly clothed and with her wrists tied behind her back in her 14th floor flat by her fiance. Nothing was apparently missing from the flat and there was no sign of forced entry.