Quote - Thanks benney got it. I'm now wondering why, having deleted half the cube, there is a stray vert in the centre of the cube?

It may not be a vertex. It may be the origin for the object. If one deletes all the vertices on an object, the origin will still remain. It's orange, and so can look like a vertex. A cube's origin, unless the cube's vertices are moved in edit mode, is in the center of the cube.