Judy, thank you so much, that was wonderful! I would never have anticipated finding so much out so quickly! I have access to all the UK censuses on Ancestry, but not US, so if it's the opposite way round for you - or anyone else - please ask.

I had heard they all started out in New Albany, so that all tied in. I gather that the 3rd child was called Richard - is that a likely reading for the difficult one in 1850? Could you also tell me please if family relationships were recorded in the 1860 and 1870 censuses?

I was expecting Eleanor's age to be 48 in 1870 rather than 58 - is that correct, do you think? I'm assuming the Cavendishes and Eleanor Kissick were still in New Albany in 1860 & 1870?

I did have a hunt on the 1880 census via familysearch:

1880
Census Place Grandview, Edgar, Illinois
Family History Library Film 1254204
NA Film Number T9-0204
Page Number 82C
Household:
Name , Relation, Marital Status, Age, Birthplace, Occupation, Father's Birthplace, Mother's Birthplace
John CAVENDISH , Self , M , 37 , OH , Postmaster , ENG , VT
Carrie CAVENDISH , Wife , M , 39 , OH , Keeping House , VA , VA
Anna CAVENDISH , Dau , S , 5 , IL , , OH , OH

It says England for father’s birthplace rather than IOM, but the age & wife’s name corresponds with what I’ve been given - unless he made the same assumption.

Census Place New Albany, Franklin, Ohio
Family History Library Film 1255016
NA Film Number T9-1016
Page Number 387A
Household:
Name , Relation, Gender, Age, Birthplace, Occupation, Father's Birthplace, Mother's Birthplace
William CAVENDISH , Self , Male , 29 , OH , Farmer , ISLE OF MAN , OH
Matilda CAVENDISH , Wife , Female , 24 , OH , Keeping House , PA , PA
John L. CAVENDISH , Son , Male , 4 , OH , At Home , OH , OH
Edgar H. CAVENDISH , Son , Male , 2 , OH , At Home , OH , OH
Charles H. CAVENDISH , Son , Male , 7M , OH , At Home , OH , OH

A Frank Shirer, who posted on this board a few years ago, was apparently a descendant of this William. He said he had a photo of his son, which would be interesting to see!

Census Place Richland, Tama, Iowa
Family History Library Film 1254365
NA Film Number T9-0365
Page Number 558B
Household:
Name , Relation, Marital Status, Age, Birthplace, Occupation, Father's Birthplace, Mother's Birthplace
Samuel GOULD , Self , M , 40 , ENG , Farmer , ENG , ENG
Elen GOULD , Wife , M , 40 , ILE-MAN , Keeping House , ILE-MAN , ILE-MAN
George GOULD , Son , S , 4M , IA , , ENG , ILE-MAN
M. A. ENSMINGER , Other , S , 23 , OH , Teacher , OH , IN
Henry CAVENISH , Other , S , 23 , OH , Laborer , ILE-MAN , VT
Amy RUSH , Other , S , 11 , IA , , OH , ---

I couldn't find John senior's widow, Mary Jane Cavendish (born about 1822, US) although she is supposed to have survived until 1904.

The only Eleanor Kissa/i/ock I found was the wife of John Kissick, although she is the right age.
Census Place Chardon, Geauga, Ohio
Family History Library Film 1255018
NA Film Number T9-1018
Page Number 43A
Household:
Name , Relation, Marital Status, Age, Birthplace, Occupation, Father's Birthplace, Mother's Birthplace
John KISSICK , Self , M , 67 , ISLE OF MANN , Tanner , ISLE OF MANN , ISLE OF MANN
Elinor C. KISSICK , Wife , M , 58 , ISLE OF MANN , Housekeeper , ISLE OF MANN , ISLE OF MANN
Anna J. COSTIN , Cousin , S , 21 , ISLE OF MANN , At Home , ISLE OF MANN , ISLE OF MANN
Willie R. KISSICK , Son , S , 17 , ISLE OF MANN , At School , ISLE OF MANN , ISLE OF MANN

Thank you once again for all your help. It's nice to see some real data to back up the anecdotal evidence.