Rookie errors! It seems at some point I have deleted the World Data block from the Outliner Tree. When I needed to create a World background I added a new World from the properties panel, however the use Nodes button under the surface tab is no longer there. Is there any way that I can reinstate a World as it would be when opening a new file? When I open Node Editor and go to Add Nodes the options are not available for selection. I changed the color to green in the properties window and see no Nodes. I presume I am missing a step.
To access the World nodes, all you have to do is switch from editing object material nodes to the world material nodes. To do so, click on the little world icon that can be found at the bottom of the node editor. It's the one located to the right of the cube icon that is currently selected (visible in the third screenshot). This will now display the world nodes.