Complex sentence: Shenton explained that the stalker had a key to Mitzeee's flat and had been watching her every move, and said that "not only has the stalker come back, but they've come back on a whole new level".
Split into simpler sentences: Shenton explained that the stalker has a key to Mitzeee's flat and has been watching her every move. She said that "not only has the stalker come back, but they've come back on a whole new level".