Here are Mary and Fanny on the 1861 census:
STEEVENS, Mary A Head Widow F 67 1794 Laundress Winscombe, Somerset
STEEVENS, Fanny Daughter Unmarried F 24 1837 Laundress Bristol
Piece: 1730 Folio: 50 Page: 17
Registration District: Clifton Civil Parish: St James and St Paul
Municipal Borough: Bristol
Address: 8, Conduit Place, St James And St Paul, Bristol
I've checked Mary's place of birth on the original images in both
1861 census: definitely Winscombe
1851 census: think Woodland is a mistranscription. Looks like Woodbr... to me. After a bit of searching (e.g. GENUKI site, Somerset pages, Winscombe) I think it's Woodborogh which was a little hamlet now swallowed up by Winscombe and not on modern maps: See http://www.visionofbritain.org.uk/place/place_page.jsp?p_id=...
Civil registration of births started in England in September 1837 and I was hoping Fanny might have been born late enough to have been registered, as the birth certificate would confirm her mother's maiden name. Sadly it seems not.
Mary's birth/baptism. Census suggests 1794/5. On the GENUKI site there are transcriptions of the parish records for Winscombe. No sign of Mary's baptism but there ARE Hall families there. GENUKI site mentions that there are also non-conformist chapels, so those should be checked out too.
Mary and William's marriage. No joy with this. Note if Mary was born c.1794 then at Harriet's birth in 1827 she would have been 33. So there may have been older children to William and Mary (adult by 1841 census) or there may have been an earlier marriages for both William and Mary.
You might want to check out the following I noted on the ancestry site:
1825 William Stevens m. Mary Radford at St. Nicholas, Bristol. I couldn't find a marriage of a Mary Hall to a Mr. Radford but you should at least check out whether Mary Radford was a widow, as the date feels very 'right' if Harriet was their first child together.
1821 William Stevens m. Mary Hall at Chagford, Devon. Place doesn't seem right, but you should check out what happens to this couple - see if they're on censuses, any children baptised in Chagford etc.
1816 William Stevens m. Mary Blackford at St. Micheals Bristol.
As William Stevens is a common name there could be others. These were the ones I found easily.
Interesting about your Henry being a mason - ws he that on the marriage cert as well?
Signing off now. Good luck!