Thank you all for your thoughts.
To answer some of the questions:
Having seen the actual record from the Seavington St Michael records (thank you again Queenie) it certainly looks like the words "Simon and" were added some time later than the baptism ( but the recorder forgot to alter the verb to 'were'). This is a reason to suspect that the vicar may not have correctly remembered the forename.
Thomas and Honor Pool(e) baptised three children in Seavington St Mary church, one in Dinnington (a chapelry of Seavington at the time) and 7 in Seavington St Michael. They lived at Meade Farm, Seavington St Mary or Michael depending where you thought the boundary was!
My 3xggrandfather was the child baptised privately on 21 April 1765 (born on 1st April and presumably sickly). He was then "named and received into the Church" (if you look at the actual record) in Dinnington on 28th April. His baptism, as Kamante has found, was also recorded in Seavington St Michael (spelt as Sim(e)on/Simmion, and mother as Hannah instead of Honor).
My 3xggrandfather Simeon was the miller at Durleigh, Somerset and died in 1848.
It therefore seems extremely unlikely to me that the child baptised with Mary in 1767 was actually called Simon by his parents.
Indeed, this child and his family, and Mary and her family, all emigrated to Canada in the 1830s or so. A family history of the Chaffey family from Somerset (who emigrated a little earlier) calls the 1767 child "Simon Joseph Poole", when speaking of the marriages of various of his children with their cousins, the Tetts (Mary's family) and Chaffeys. See
https://www.yumpu.com/en/document/view/15626085/download-complete-pdf-file.
It also confirms that Simon Joseph married Ann Niess in Chillington in 1800 when he is actually recorded as just Joseph (and similarly for the baptisms of all his children in Chillington). When his sister Eunice married in 1790 a witness was 'Joseph' Poole. The family bible of Joseph Poole and Ann's (née Niess) descendants mentions their ancestor's marriage in Chillington in 1800 and arrival in Canada.
(Simon) Joseph died in Ontario, Canada in 1825. His will, if he made one, might confirm his given name(s).
In addition, having named their first children after grandparents it seems to me highly likely that Thomas and Honour then named a couple of sons after Honour's own cousins, Simeon and Joseph, since neither of these forenames were traditional paternal or maternal names.
Sorry for the long explanantion; hope it clarifies my thoughts.
