Complex sentence: Frederick William Frohawk based his restoration in the former book on an outline sketch, which was in turn based on a 17th century sketch drawn by Sir Thomas Herbert, which is now known to depict the Red Rail.
Split into simpler sentences: Frederick William Frohawk based his restoration in the former book on an outline sketch, which was in turn based on a 17th century sketch. This sketch, drawn by Sir Thomas Herbert, is now known to depict the Red Rail.