Just to note that it seems fairly well established that both Thomas 1798 and James 1788 were sons of James Couch and Dorothy.
http://www.rootschat.com/forum/index.php?topic=472835.msg4968734#msg4968734They married in 1829 and 1827, respectively, in Lincolnshire.
There is a Dorothy Couch death March quarter 1846 in Torrington reg dist Devon, which covers Langtree. Have you looked into that? I can't find anybody who looks like that name in the 1841 census, even trying wildcards and obvious mistranscription possibilities (like Conch ...).
edit - found her via familysearch, which has the person who must be that Dorothy Couch in the death record as Dally Couch. Ancestry, bless its heart, has her as Dally Conih. Aged 90 (c1751 so a tad old, but 90 is a bit of a round number), pauper, in Langtree, living with Dally Thomas, 60, likely a widowed daughter.
Aha. Dave Jones's post in the previous thread said this already:
"He was the son of James Couch, miller of Langtree, Devon, who died on 28th March 1841, and Dorothy who died at Langtree in early 1846 aged 95."
The James Couch death reg in Torrington reg dist March quarter 1841 is also in the GRO index, and that certificate might be even more likely to give useful information about him.
Starting a new thread without at least giving all known or possible relevant info, not the best idea!
A couple of the on-line trees at Ancestry/Mundia do state a sister Dorothy born 1781 (date could be taken from that 1841 census).
The 1846 Dorothy Couch death certificate might mention her widow status and husband.