Complex sentence: The Italians had been wrongly informed that the Mediterranean Fleet had only one operational battleship and no aircraft carrier but there were three battleships and a damaged British aircraft carrier (HMS "Illustrious") had been replaced by HMS "Formidable".
Split into simpler sentences: The Italians had been wrongly informed that the Royal Navy's Mediterranean Fleet had only one operational battleship and no aircraft carrier. In fact, there were three battleships and a damaged British aircraft carrier (HMS "Illustrious") had been replaced by HMS "Formidable".