There were two John Trethewey baptisms at St. Blazey in 1806.

1. 9 Feb 1806 to William and Catherine (almost certainly nee Scolier) Trethewy
2. 25 May 1806 to William and Mary (maiden name not yet known) Trethewey

There was also a John Trethewey - Eliza Paul marriage at St. Blazey 26 Jul 1834.

And there were many John Trethewey baptisms at St. Stephen in Brannel in the same period (1806 - 1811):

1. 27 Dec 1806 to Robert and Elizabeth
2. 14 Jul 1810 to John and Mary
   
3. 6 Aug 1810 to Thomas and Jennifer
4. 1 Sep 1811 to John and Mary
   
5. 28 Mar 1812 to Thomas and Elizabeth
6. 13 Sep 1812 to Richard and Rebecca

Thus, I cannot tell which John Trethewey was the husband of Eliza Paul.