Further to my above ramblings, and thanks to the excellent Hackman Series of Hampshire Bishops Transcripts
(see
http://prtsoc.org.uk), this is what I think might have been happening with the Fielders of Wellow.
In 1786 Benjamin Fielder married. According to the Bishops transcripts, he married Ann Allen, but he's always shown with wife Elizabeth, so I think this is an error

In 1787 James Fielder married Ann Allen
Benjamin and Elizabeth had (at least) the following children -
Sarah - baptised 1788 aged 13 months
John - 1791
Mary Ann - 1793 (married Moses Rogers of Sherfield English 1815)
James - 1795 (married Martha Hood of Plaitford 1815, died 1825)
William - 1798
Joshua - 1801
Charles - 1803
James and Ann had the following -
Sarah - died 1791
John - 1793
Elizabeth - 1795
Edward - 1797 (married Mary Ann Cooper - Sherfield 1827)
William - 1799
Sarah - 1801
Robert - 1803 (married Sarah Penny 1825)
Mary - 1805
Dinah - 1808
I reckon it's quite likely that Benjamin and James were brothers. A look at the parish records proper may give Benjamin's wife's name, or may not! Because West Wellow parish records are now held in Hampshire Record Office, not Wiltshire as I thought, I haven't been able to look at them.
Your John Fielder and my William Fielder could both come from either family since they seemed to have the irritating habit of giving their recently born children the same names! I've found a marriage for one william and a death for the other, but still can't sort out which one was my gg grandmother's father

How does all this compare with your researches? Any extra information you have will be very gratefully received

Best wishes
Angela