Complex sentence: The Metropolitan Police had received a three minute warning but were unable to evacuate the station completely before the device exploded, however the British Transport Police were able to evacuate much of the area just prior to the explosion, thereby reducing casualties.
Split into simpler sentences: The Metropolitan Police had received a three-minute warning but were unable to evacuate the station completely before the device exploded. The British Transport Police were able to evacuate much of the area just prior to the explosion, thereby reducing casualties.