Quote - ok, being a total newbie with PP2012, if i make the dome invisible to camera and raytracing but leave the light emitter box ticked, does Firefly still use the dome and its image in IDL calculation? 

ie i need the light effect, but want to overlay the render onto a BG of my choosing.

Sounds like you want just a plain old image based light, for that you do not need to use a skydome or similar; it does use a different map-type, though.

As for the IDL/2012, i do not know, but why not try it out? Just disable all other lights, leaving only the skydome on and render it, if you get anything lit, it works :-)