Running "Containment" for multiple regions

I'm trying to identify the overlaps between a random array of metaballs and a grid of points. This works fine when the metaballs congeal into a single mass but as soon as there are two or more the 'Containment' function stops working.

I think its something to do with the way I'm structuring my lists but I can't simply separate the list items out and run them one by one because;

a. depending on the way the blobs connect I could have anywhere between 2-100

and

b. I'm trying to find the area outside the blobs and separating them out means each set of 'outside' points just cancel each other out.

This is what I have set up at the moment, which works for one metaball blob. Any ideas on how to get this working?