It would appear from parish registers that Mary Ann's parents were married when she was conceived. It would certainly be interesting to know where that tale originates.
Where were William and Sarah in 1841

Records of Hartfield and adjoining parishes seem to infer that there were many families with the surname Heat, no 'h' on the end

There are two baptisms in Withyham for a Sarah Heat 1773 & 1776, perhaps it was the one of those that married Joseph Turner