Ok, time for my stupid question... I'm using Repetier-Host, but I can find nothing that will allow me to separate parts... I'm using a Mac; maybe that Mac version doesn't have that feature?
Ironically, netfabb (basic) has queried me in the past when opening .stl files, that there were separate objects, should it separate them; but not for this plate. :(
I'll look into some of the other options. Thanks.