Thanks WP, but I am not on the mailing list. So I did some more diggings and found a better solution, maybe one that will interest other users.
The translate unit does try and open its own translate file (as well as the application's one) but it does not try in as many places. I found that on Linux, its tries only -
/usr/share/locale/es/LC_MESSAGES/lclstrconsts.mo
Unlike its search for the application's file, where it first looks for $APP_NAME.po in several places and then $APP_NAME.mo in the same places. So, this means that the sensible thing to do is separately convert both the LCL file and your app's file to .mo and put them in dir above.
I have not yet established where it looks for the LCL file on Windows and Mac, when I have done so, I will add a note about this behaviour on the wiki page.
(Note : the wiki pages currently talk about lcl.mo, strace assures me on linux at least, thats never an option.)
Davo