Complex sentence: Lucas's preparations in the Netherlands had been observed by British spies and reported in January, more than a month before the expedition sailed and the frigate HMS "Carysfort" was sent to the Cape with a warning, arriving in April.
Split into simpler sentences: Lucas's preparations in the Netherlands had been observed by British agents and reported in January, more than a month before the expedition sailed. The frigate HMS "Carysfort" was then sent to the Cape with a warning, arriving in April.