Joseph Downing: How My Family Came To Essex

My majority of my research has centred on my mother's family, the Downings, who lived in Southminster, Deepest Essex (and later Southend-on-Sea).  This lively family were always up to mischief and rarely off the pages of the Chelmsford Chronicle.  But I knew they weren't always from Southminster.  Every Downing that I had researched was descended from the same man, Joseph Downing.  According to his burial record, Joseph was 77 when he died in 1819, meaning he had been born around 1742.  But there was no baptism for Joseph in Southminster, or indeed of any other Downings prior to him.  Where did he come from, and why?