Great extension!

Slicer5 is a bit more picky than SliceModeller regarding Solid Manifold requirements.

My model is a STL exported from another program called BoardCAD. It has 900 short edges, 66 internal faces and 3 surface borders which I will need to correct to use this plugin. SlicerModeller doesn't need me to do this. sliced with slicemodeller that couldn't with slicer5

If you are trying to correct a model I suggest to use MeshMixer because it has many functions specifically designed for this purpose. I spent weeks trying to fix these mesh to solid problems (SOlidSolver, FixSolid, SolidInspector) until I found this! Also bear in mind that many tutorials for it are out of date - the tool now is much better. Bear in mind that it's designed with mm scale in mind... whereas sketchup needs metre scale really due to it's own shortcomings. Still, you can export and import with STL import/export sketchup plugins and work between the both.

This looks like an important plugin.

It seems one thing it could be used for (correct me if I'm wrong) is for fixing holes in a solid identified by SolidInspector... but which tool is appropriate...

Here I'm trying to plug holes in the underside of a surfboard... it has had the poly count reduced for testing. I want to smooth over those holes: