Actually, Huia, you have to forgive me - I had completely disregarded what you said in your OP:
"My gg Grandfather is Edward Saunders Clark. He was christened 27 Feb 1831 in Malborough Devon. No father mentioned. Mother Elizabeth Clark."
It's just that there is such a shortage of Edward Clarks, I found about the only one going in 1841! And he is in the same place as the one you were looking at in 1851, Loddiswell, and the 1841 one doesn't seem to be anywhere else (there is no death 1841-1851, for example). The Edward Clark who married, coincidentally, Harriet Wills Mitchemore in 1862 in Kingsbridge reg dist could be that 1841 Edward Clark. Although that couple seems to be in Aveton Gifford in 1871, and that Edward was born c1819 ... (he was a widower born c1821, in 1861).
The idea that Saunders was his father's surname is of course the most sensible theory. What is very possible is that in 1841, he was recorded under a stepfather's name, if his mother had married.
I'm just going to rule out one possibility.
In 1851, Edward Pratt born c1833 Malborough is a servant in Malborough at the farm of George Hodder. In 1841 he is a servant in Malborough on the farm of Philip Turtley. But I would say he was Edward Ash Pratt and he married in 1854 and died in 1921 aged 90.