In case you were wanting a check on the information about your mother and her family (her father is George in the 1911, it helps to know) -- her birth was registered in March quarter 1911, in Honiton registration district (where they are living in 1911). Honiton covers Ottery St Mary and Cotleigh (spelled with one 't'! as it is on the 1911 census form):
http://www.ukbmd.org.uk/genuki/reg/districts/honiton.htmlI don't quite understand the 1911 census, which says "Cotleigh Ottery St Mary", since Cotleigh and Ottery St Mary are actually on opposite sides of Honiton. I think the way I see it transcribed (in two places) is probably correct (although I don't know where it came from on the form): the address was
Cotleigh Cottage, Ottery St Mary. So they didn't actually live in Cotleigh, but rather in Ottery St Mary.
(I understand there is some rule against 1911 census lookups here. This isn't a lookup, as bobpen already had the information: it's me trying to sort it out for myself in order to look further back.)
In the 1881 and 1891 censuses, Robert George's parents are Robert and Ann. Father Robert was born c1852-3 in Whimple, Devon (where they are living in 1891).
Unfortunately, father Robert is not with his own family in the previous censuses. In 1871 he is a servant, and in 1861 he is with an uncle in Ottery St Mary: William Dunn, a tailor, born c1823 in Whimple. Two of Robert's sisters (at least, two nieces of William) are also there: Jane c1855 and Harriett c1859. This does suggest they had been orphaned. (It is possible that Robert c1862 was the son of unmarried parents and had his mother's surname, but since he appears to have two sisters as well, this would seem unlikely.)
In 1851, William c1823 (the uncle of Robert c1852, Robert George's father) is with his parents William, born in Uffculme, and Jane, and has brothers Robert c1834 and Thomas c1839 and sister Mary c1837, and there is a grandson John J aged 1.
The family is in Whimple in 1841, with father William c1791, a tailor, and no sons other than those shown in 1851. However, the couple in 1841 could have had other sons who had left home by then. For example, there is a Samuel Dunn c1816 nearby in Whimple who is almost certainly another son of theirs, since he is the only other Dunn in Whimple. He might be the most likely candidate for father of Robert c1852.
I might have got my Dunns muddled there, but I hope not! You would need the birth certificate of Robert George's father Robert, to follow the trail back accurately. His birth seems to be registered March quarter 1853 in St Thomas reg dist, which covered Whimple.
So -- it looks like you do descend from a William Dunn born c1791 per the 1841 census -- but a different one from the William Dunn in the opening post. Very likely related, though?