Complex sentence: South American gauchos who settled in the 1840s and 1850s, and since the late 1830s and early 1840s onwards settlers from Britain and other countries including various European nations, most recently with especially significant contributions from St. Helena and Chile.
Split into simpler sentences: South American gauchos who settled in the 1840s and 1850s; and since the late 1830s, settlers from Britain and other countries. These latter include settlers from various European nations and most recently, significant contributions from St. Helena and Chile.