Allan,
This is my take out, simply from accessing FamilySearchRecordSearch for Backford, only:
Firstly, I don’t know much about Cheshire Land and Tax Assessments or when it was introduced, but I note that Ralph was liable in 1778 in Backford, which may suggest a possibility he was new to the area?
Secondly, I also note that following on from Ralph’s death on 13th Feb 1800, Mary be came liable to Cheshire Land and Tax Assessments (1803)- would this not indicate a tie of Mary (died 1808) to Ralph?
Thirdly, if Mary/Ralph had Elizabeth (1776), Peter (1779) and Daniel (1782) in her late 30s and 40s, the likelihood is that she would have had children before then?
Fourthly, I can find no other reference to Ralph, but there do seem to be coincidences between many of the children of Ralph (1727 Over), and Mary Trelfa (Married 1758 Over) baptised in Middlewich and their namesakes found in Backford after 1776.
William 11 Jun 1758
John 27 Jan 1760
Thomas 7 Oct 1761
Ralph 16 Mar 1764
Joseph 27 Nov 1765
Mary 13 Nov 1767
Richard 16 Apr 1770
Hannah 29 Apr 1772
Samuel 4 May 1774
(Mary’s date of birth is variously listed as around 1730, or 1740 or possibly even 1736)
I’ll discuss these in turn:
William. William oakes / Margaret Griffiths 12th october 1779 and there are children born to them. Also a William died25/51826 in Backford.
John. Can’t find much about John- there is a Cheshire Land and Tax assessment 1795, but could this be for John born to William/Margaret in 1782?
Thomas. Tom Oakes and Margaret had a son, Tom Oakes born to on 30/12/1787; there is a Thomas who died 7/9/1817.
Ralph. Not much about Ralph at all; could he be father of Hannah, born and died 1790?
Joseph. There are children born to Joseph and Anne Oakes around 1790 and a Cheshire Land and Tax Assessment 1789.
Mary. Marriage- William Williams / Mary Oakes 9th Nov 1790 and there are children born to them.
Richard. Only thing I can find is a burial 26,12/1774 at St John the Baptist, Chester.
Hannah. Do you have any record of Hannah (1772), Allan? Did she die?
Samuel. Marriage- Samuel Oakes to Jane Rowlinson 2nd September 1797, Backford. Had children.
Fifthly, As you say, Ralph Oakes’s (17/7/27) parents may be Ralph Oakes (baptism at St Chad's, Over, 14 Jun 1702, father John Oakes) and Mary Holland (St Chad’s 1804?) who married at St Oswald , Chester church on 14th feb 1726.
Allan, I am but a novice in doing this sort of thing, and would be grateful if you could add to, correct or point out erroneous inferences. However, if correct, there is strong circumstantial evidence of connections between families at Backford and Middlewich.