From the information you have given, it looks as though you have looked at censuses (possibly on the Brew website) and the IGI.
It is difficult to be certain of relationships during the eighteenth century unless you also look at a number of wills, as well as the parish registers [both available on film through the LDS Family History Centres].

You say Samuel was b.1762. On the IGI this entry is given as son of John Brew. Was a place given at bapt? Were places given at the various marriages? Or for the Cannell parish register entries?

Wills which look as though they might give crucial information include:
BREW, Samuel 1827 Lon A 2 0106245
BREW, John 1783 Lon A 1 0106228
BREW, John 1791-1 Bra E d 0106402
BREW, John 1800 Lon E 2 0106407
BREW, John 1816 Lon E 2 0106418
MAXWELL, Thomas 1826 Onc A 7 0106244
[From Brian Lawson's wills index.]

I understand that these films arrive fairly quickly in Canada, and when transcribed you could add them to Brian's web site.
Good luck,
Sue