FamilySearch shows baptisms to James Couch and Dorothy in Buckland Brewer, about 4 miles from Langtree:
James 1788 *
Margaret 1786
Elizabeth 1791
Susanna 1793
Joseph 1793
(They are the only Couch baptisms in batch C05004-1.)
These could be siblings of Thomas.
* Indeed, in 1851, James Couch, born c1789 in Buckland, Devon (transcribed by Ancestry as "Ruckland") is living in Claythorpe, Louth reg dist, Lincolnshire with his wife and children, all born in Lincolnshire.
James Couch married Sarah Cook, 1827,
Goulceby, Lincolnshire.
https://familysearch.org/pal:/MM9.1.1/NKZX-TXMThey are the only two Couch marriages in Goulceby at familysearch.
James and Sarah had daughter Ann, baptised 1827 and buried 1829, in Goulceby:
https://familysearch.org/pal:/MM9.1.1/NGHQ-RV3I would venture to guess that it was the parents of Thomas who moved the family to Lincolnshire.
There are several Ancestry/Mundia trees with Thomas in them, one at least showing parents James and Dorothy and brother James.
Maybe Bilham has managed to find all this by now -- but in any case maybe it will help Marion!