Thanks for the additional information. Based on what was in Wellington's Dispatches concerning the provisioning the Portuguese troops, it appears that the St. Jago is the Santiago that is near Seia (called Cea in the Dispatches.) Supporting this is that there is a Sandomi and Sta Marinha within 10 km of Seia and Sampayo (Sāo Paio) is only 22 km. Although there is a Sampaio close to Viseu. . . about 50 km from Seia.
I have not had time to research where the main road from Coimbra to Celorico to Guarda was in 1810. Is it the A25 that goes through Viseu or the N25 that goes through Seia? I suspect that it was the A25. Although Seia is not on A25 the troops were probably there because of the lack of other places for their cantonments closer to A25.