I found a solution using xrandr's size, though its a bit of a hack.
xrandr --size 1024x768
on its own doesn't work, but if I make the --size smaller then back it the size I want it works.
So I adding this code at the top of openbox autostart
##make area desktop right size - ie the size openbox think the screen is
# hack - just setting correct size doesn't work
# set smaller wait a second - allow xrandr to work, then set bigger
(xrandr --size 800x600 && sleep 1s && xrandr --size 1024x768) &
this is in addition to adding the 10-monitor.conf file (see above).
now I can get on with using manjaro