Complex sentence: Clearing the mines and the ships sunk by them was a slow process; "Illustrious" did not reach Suez Bay until 20 March, and sailed for Durban, South Africa to have the extent of her underwater damage assessed in the drydock there.
Split into simpler sentences: Clearing the mines and the ships sunk by them was a slow process and "Illustrious" did not reach Suez Bay until 20 March. The ship then sailed for Durban, South Africa to have the extent of her underwater damage assessed in the drydock there.