j meyer & Mr. Lamppost: Thankyou both for the help. It turns out that the surface layer works a treat but not the way you suggested as a child layer, J. (I'd tried that, and couldn't get the right result, and the merge shader approach too, the white parts were also merged with the first input that way). I've simply replaced the default shader in the caustics group with a surface layer. Plugged the inverted voronoi into the luminosity function(as I only want the voronoi 'casting light') and into it's breakup shader too.
A low coverage(0.1) in the breakup tab coupled with high levels of luminosity(3) creates the effect I was after.
I can't beleive I missed thinking of the coverage level in a surface layer but, to my defence, I did think that the masked areas should have been invisible before reaching it, using the default shaders opacity levels, it certainly appears so in the middle image I posted up there. ^^
If the opacity slider works for an image map with an alpha image, why won't it behave correctly to a black and white function output? Instead, the unspecified areas(Black in an image map) where colour won't be applied, is applied as black anyway. I wasn't looking for transparency, just an opacity mask.