Im trying to create a complex script which will analyse each of the vertex of a mesh and establish the vertices that are linked to that single vertex.

I was wondering if I took say index 5 through to 10 of a vert array, would taking the same 5 to 10 in a triangle array grab the correct triangle allowing me construct a small section of the original mesh?

Or am I completely misunderstanding the way meshes are constructed in unity?

This depends on what you mean by ‘taking the same’

The triangle array contains indices into the vertex array. You cannot count on low-numbered vertices to be the first indexed by the triangle array. The first triangle in the array may be: [ 501, 1022, 44, …

So if you want to take part of a mesh, you should look first to the triangle array. If you want a patch of triangles, you’ll need to search the triangle array for matching edges (triangles that share two of the same vertices) and make a list of those.