Hi terrymac, and welcome to Rootschat.
These Newells are a bit of a pain, aren't they?
I am trying to lay claim to the William Newell s/o Adam & Ann (Redding) who was baptised in Great marlow in 1760. So this would be your Mary's brother.
My William Newell married in Hitcham, Bucks in 1785 but I'm struggling to find his baptism - it certainly wasn't in Hitcham. The Great Marlow baptism fits pretty well with his age at death and the 2 places are not far from each other.
I'm still not sure where the father, Adam Newell sprang from if the burials in W Wycombe are correct.
However, just to throw another spanner in the works (sorry!) I'm not sure how Mary & William's father Adam Newell could be on the W Wycombe census in 1760 if he had the children I've found:
1757 John in Great marlow
1759 Mary in Gt Marlow
1760 William in Gt Marlow
1761 Henry in Gt Marlow
1764 Jenny in Medmenham
1765 Ann in Medmenham
1767 George in Medmenham
1768 james in West Wycombe (died 1770)
Also, I've found a 1768 Settlement Certificate issued from Medmenham for Adam Newell, his wife Ann and 6 children who were living in West Wycombe.
Also, a Removal Order of 1781 issued in Medmenham for the removal from Little Marlow of Adam Newell, Ann his wife and their children Jenny about 18, William about 17 and Joseph about 15 (could this be an error and should be George?).
Finally, I have a burial for Adam in Medmenham on 27 Dec 1815 age 82; I haven't found one for Ann yet. If my maths are right, that would make Adam's birth year almost on the nose to be the 1736 one (if he hadn't been buried!)!!!
Altogether bamboozled!!!
Jill