Complex sentence: Prior to the 1780s, most of the fine quality cotton muslin in circulation in Britain had been manufactured in India; thanks to advances in technique, British "mull muslin" was able to compete in quality with Indian muslin by the end of the 18th century.
Split into simpler sentences: Prior to the 1780s, most of the fine quality cotton muslin in circulation in Britain had been manufactured in India. Due to advances in technique, British "mull muslin" was able to compete in quality with Indian muslin by the end of the 18th century.