Complex sentence: The artillery batteries gradually shortened their range until the start line for the creeping barrage was reached and then at 0310 the main barrage began with flanking smoke screens laid down by the artillery and trench mortars.
Split into simpler sentences: The artillery batteries gradually shortened their range until they reached the start line for the creeping barrage. Then, at 03:10, the main barrage began with flanking smoke screens laid down by the artillery and trench mortars.