Complex sentence: Catesby and the others, "en route" to the Midlands, were alerted to his arrest by those conspirators who had since fled London, and together they rode to Dunchurch and met Digby and his party.
Split into simpler sentences: Catesby and the others, "en route" to the Midlands, were alerted to his arrest by those conspirators who had since fled London. Together they rode to Dunchurch and met Digby and his party.