Thanks Wolfgang.

I've been playing with different bodypart and backpart setupsectionblock settings: number=yes/no, page=yes/no/right, before/after={\page[left/right]}]

From what I observed and understand (from the manual and http://texshow.contextgarden.net/command/detail/296 )...

number=yes/no : doesn't have any effect at all

page=yes : starts (a section block on) a new (left or right) page
page=no : doesn't have any effect at all
page=right: starts (a section block on) a (new) right page

before/after : before/after starting the new page

\page[left/right] : forces a left/right page

The following

\setupsectionblock[backpart][before={\page[left]}]

gives more what I was looking after, however there's still
a blank numbered page as the last chapter's last page
which I've unsuccessfully been trying to get without page
number.

Alan