Complex sentence: Belhaven claims to have begun brewing in 1719; at that time it had more than 24 small and large competitors in Dunbar, Belhaven and West Barns.
Split into simpler sentences: Belhaven claims to have begun brewing in 1719. In the first half of the 18th century Belhaven had more than 24 small and large competitors nearby in Dunbar, Belhaven and West Barns.