I think it's a good idea to get the fiche yourself if you can possibly afford it

You can't beat going through registers yourself to pick up on context and margin notes etc.
I did have a quick look for a birth for Levi but couldn't see one in Hardington Mandeville, nor were there any banns there that might have related to his marriage. Interestingly, the Lewis Dalamount who was having children about the same time and Levi and Grace, was married in Hardington Mandeville to an Elizabeth Sibley. The same names just keep appearing....
What you said about Ann's age in 1851 does make it very likely we have the right one. The 1841 ages were rounded
down to the 5 year mark below their current age, so if Ann was born anywhere from 1782 to 1786, the year of birth would show as 1786 on the 1841, so that matches nicely.
Good luck with your researches

Best wishes
Angela