Complex sentence: By 1751, he had moved to the headquarters of British horse racing, Newmarket, and had become trainer and jockey to fellow Yorkshireman and future Prime Minister, the Marquis of Rockingham.
Split into simpler sentences: By 1751, he had moved to the headquarters of British horse racing, Newmarket. In Newmarket, he became trainer and jockey to fellow Yorkshireman and future Prime Minister, the Marquis of Rockingham.