OK, I have now tried this:

1. object obj in meshlab
2. Go to Filters -> Remeshing, Simplification and Reconstruction -> Simplification: Quadric Edge Collapse Decimation
3. Enter target of 1000 faces
4. Export
5. Use Michael's obj23dm converter, choose closed polygon for each face
6. Load into MoI

OK, looks good. A lot less clutter. But the "surface" is just a collection of triangles (closed curves) and there are no faces.

So what I want to do now (for example) is create a NURBS surface over the top, maybe just part of it, and then extrude that. But how?

Thanks, Andy