Complex sentence: As they leave for the railway station, Allan informs Quennell that Dr. Mallinger did in fact know Frederick Britewell, prompting Quennell to perform an immediate search of Mallinger's home, only to find that the scientist and his daughter have left for Upper Higham.
Split into simpler sentences: As they leave for the railway station, Allan informs Quennell that Dr. Mallinger did in fact know Frederick Britewell, prompting Quennell to perform an immediate search of Mallinger's home. He finds that the scientist and his daughter have left for Upper Higham.