Complex sentence: Frederick Leach's most significant original commission was probably the interior decoration of St Clement's Church, Cambridge, where he was responsible for the entire interior decoration, which included some elaborate wall paintings that he personally devised and executed.
Split into simpler sentences: Frederick Leach's most significant original commission was probably the interior decoration of St Clement's Church, Cambridge. Here he was responsible for the entire interior decoration, which included some elaborate wall paintings that he personally devised and executed.