Hi again. I have relooked at your names and find this is what I have going backwards -Arthur Porter b 1874 Sth Australia, ( siblings are Lavinia, Thomas, Mary, Wilfred, Herbert, Jasper, Muriel)Is this the same Arthur of yours?
Arthurs father is Jasper Porter mother Elizabeth Nankivell from Cornwall, m 27 Nov 1871; Jaspers parents are Jasper b 1812 married to Mary Parker, Moorlinch, died 31 Mar 1896; Jaspers father is John Porter bap 1773 m. Edith Turner b. 1774 Moorlinch; in 25 Apr 1798.
Now this could be the connection after all that you are looking for. My ancestor could be your Jaspers  brother. My email address is coregatxtra.co.nz if you would like to contact me and I can send you some trees and information that might help.
I have not been able to get back further than John 1773 but there just might be a connection to a Joseph Porter of Edington just on the other side of the Poldens [abt1745] who married Hannah someone whose children were named Joseph, Sarah, Thomas, John, Jasper and possibly a Hannah. I only have descendants of Joseph and John Porter. John Porter of Moorlinch leased farm land at Edington as well as Catcott.
I have been in touch with descendants of John Porter 1803 whose relation was a brother to my gt grandfather Henry Porter. Some of this Josephs descandants ended up in Australia too. Joseph and Henry had two other sidblings Thomas and Edith Mitchell Porter who I think died in infancy. I think Thomas married eliza Reeves frn Ashcott. They didn't seem to have any children.
I shall hunt out the tree that John of Taunton sent me.
Katherine O'Regan NZ
Look frward to hearing from you again.