I found a transcription of the High Littleton registers on-line - have you seen them ?
http://www.highlittletonhistory.org.uk/registers.html
1800 Jun 1 Nehemiah s Isaac SAGE & Elizabeth
1802 Oct 17 Joannah [B/T Johanna] d Isaac SAGE & Elizabeth
There are also baptisms for children of Jacob, Jonah, Jesse and Nehemiah SAGE.
So the 1799 marriage is possible, just a long way from home. Couldn't see a baptism for Isaac though.
Looking at the "strays " burial section, a lot of High Littleton people were buried at Cameley (eg Jonah Sage aged 78 in 1847)
Di