Many thanks for your efforts Alan. It's been a little complicated because this Isaac had a son Isaac, christened in 1787, whom I originally thought was most likely the father of my 4th gt-grandmother Martha. South Australian records showed that she was born in 1812 in Wiltshire, and I have her christening at Brokenborough to parents Isaac and Sarah.
Having established that Isaac junior (1787) married and settled with his large family in Churchdown, Glos., where his family did include a slightly younger Martha, I had to have another look at Isaac senior (1755). It only works if his first wife Alice had died, but that may be difficult to prove. He must also have been a healthy specimen, who would have been 63 when the third and last child was born to him and Sarah in 1818.
An earlier searcher had given me a burial date of 12 June 1828 for Isaac senior, which is a year later than the one you have quoted. Would it be possible for you to double-check that for me? It's not on Ancy, and your 1827 makes Sarah's remarriage in October 1828 look a little less hasty.
Kind regards,
Didy