Complex sentence: They won their heat and so went onto compete against Jenni Falconer in the semi-final on 20 March, which they also won, which meant that they made it through to the final the following day where they competed against Sally Gunnell.
Split into simpler sentences: They won their heat and went onto compete against Jenni Falconer in the semi-final on 20 March. They made it through to the final where they competed against Sally Gunnell.