Mary Mitchell's page is a blank at the moment, no clues/leads at all.
From Little Marlow Parish Registers:
24 Nov 1771 Robert (X) MITCHELL
Mary (X) CLARKE, both of this parish
married by banns called 13, 20, 27 October 1771
Witnessed by: William MITCHELL, John (X) CLARK
24 Mar 1773 Robert MITCHELL
buried8 Aug 1773 Lydia dau of Robert and Mary MITCHELL
baptised22 Oct 1798 Charles (X) CROUDY of this parish, bachelor
Lydia (X) MITCHELL of this parish, a spinster
married by banns
Witnessed by: Samuel ADAMS, Wm CLARK
So it looks like Mary MITCHELL was formerly Mary CLARK
I did not have time to search the burials although I know there are some
From a transcript of the Little Marlow Parish Registers held at the Centre for Buckinghamshire Studies in Aylesbury:
BURIALS
11 Mar 1768 Mary HASTINGS, widow
22 Oct 1789 John HASTINGS, an infant, a pauper
9 Jul 1791 William HASTINGS
18 Jul 1791 John HASTINGS
15 Nov 1796 Mary HASTINGS
1 Feb 1798 Ann HASTINGS, a child
18 Jun 1811 John Thomas HASTINGS, an infant
26 Dec 1812 William HASTINGS, an infant
BURIALS (Original register Ref: PR141/1/15)
17 Dec 1818 Mary, wife of John HASTINGS of Well End, aged 70 years
7 Feb 1836 John HASTINGS of Workhouse, aged 79 years
So Mary CLARK has an approximate year of birth of 1748 and John HASTINGS an approximate year of birth of 1757. Looks like he could be the one from Cookham, especially as a widow Mary HASTINGS was buried at Little Marlow in 1768 who just might be his mother!