Since my last post I've been able to trace both a parish record and an entry in the GRO (General Registry Office) index for a Sarah Wybrow* who died in 1841. Her death was registered in the July-September quarter at Lexden, Essex, and her burial is recorded on 11 July 1841 at St Mary's church, Bures St Mary, Suffolk.
The parish record, (a transcription from the Suffolk Family History Society, provided on a commercial family history site), gives Sarah's date of birth at 1806. This would certainly fit with the dates for Sarah Baldwin. It wouldn't be unknown for someone to be buried in a place they were strongly associated with, rather than in their local parish church, especially since Wormingford and Bures St Mary are so close.
It would also explain why Sarah had no children after David's birth, despite being so young. From the parish records it looks as though the Sarah Whybrew whose death is registered in Lexden in 1847 was born in 1770, which helps to exclude this date.
*The names Whybrew, Whybrow, Wybrow, Wibrow etc seem to be used interchangeably at this time, so the difference in spelling of Sarah's name is not of great concern.