Complex sentence: Arnold was the first to produce marine and pocket chronometers in significant quantities at his factory at Well Hall Eltham from around 1783, during the next 14 or 15 years he produced hundreds before he had any kind of commercial competition.
Split into simpler sentences: Arnold was the first to produce marine and pocket chronometers in significant quantities at his factory at Well Hall Eltham from around 1783. During the next 14 or 15 years, he produced hundreds before he had any kind of commercial competition.