Thanks. Yes that article had already helped. It led me to the /etc/default/keyboard file
which I edited to include the variant I wanted. Works a treat! but as I said earlier,
only outside of XFCE. As soon as that is restarted it's back to the old default setting for XFCE
with no variant.
But not to worry! Late last night I found a reference in the Arch Linux wiki which put me on to the answer:
"The most obvious and easiest way (to modify Xconf settings) is to go to "Settings" in the main menu and
select the category you want to customize.
However, not all customization options are available this way.
Customization can be done completely
from the command line using the program xfconf-query
. See the XFCE online documentation. http://docs.xfce.org/xfce/xfconf/xfconf-query
...and they're right. It's all there. Just a question of finding it