Thanks for all your efforts trish1120 and rosie99.
Let me explain what I know for sure:
Thomas and Honour (Honor/Haner/Hannah - you have to imagine the scribe was hard of hearing and/or listening to the rural Somerset accent !), married 1755 in Kingstone, Somerset, settled in Seavington St Michael and baptised first 3 children in Seavington St Mary (1 mile away) 1756-1763, child n° 4 in Dinnington (a chapelry of Seavington at the time) 1765, and then 7 more in Seavington St Michael 1767-1781. My ggggrandfather is n° 4, and Samuel n° 10.
N° 4, Simeon, was a miller in Durleigh, Somerset (died 1848). He lived with Mary Browning (1776-1836). There were 6 children baptised in Durleigh, all given the name Poole, 3 altogether in 1810, then 1812, 1815 and 1817.
I'm looking for Samuel because of this vague possibility that Mary had married him, left him and therefore she and Simeon couldn't marry because it was not known if he was still alive. And I'm wondering in fact whether the 3 children baptised together (aged 7, 3 and 1) were in fact Samuel's??
However it seems clear that the only Samuel Poole and Mary Browning couple we can find (in Manchester, via the son Thomas' immigration record) and who had at least 2 sons (Samuel and Thomas) - can't be the Samuel and Mary I'm looking because of their birth dates (c. 1769), places of birth (Lancs), and their being together still in 1841.
The names seemed such a coincidence....
Another angle as to why Simeon and Mary didn't marry is of course that Simeon had married another girl somewhere (abroad perhaps?). In fact I know nothing for sure about his whereabouts between 1779 and the baptisms in 1810 (the family has always assumed that Simeon acknowledged all the children as his as they were given the name Poole - but then, I thought, that would work for being the brother's children too.)
Oh well... don't think I'll ever solve this mystery.
Margaret