Basically, when I switch from Blender Render to Cycles and back to Blender Render, my textures do not render. I can see them in material view and texture view in the 3d viewport, just not upon rendering. I had not set up a node tree in cycles, I literally just switched. I tried deleting all my materials, getting rid of the data and I have then re-assigned materials and textures and set them to the UV’s (including in the UV editor) etc etc. This should all work. If I open a new project and do it, it works, just not in the one where I have switched between render engines. What have I done wrong / broken, is this a bug? How do I fix this?
Check your materials are set up correctly.
Cycles and Blender Render materials are not switchable between the two renderers. If you change renderer then you need to remake your materials.
I had not set up a node tree in cycles
If you created a cycles material, which you must have if you had a texture, then you set up a node tree, though not necessarily in the node editor.
Don’t go chopping and changing between the two renders unless you know what you are doing, you’ll just come to grief
No blend file supplied with your post so we are unable to check what you have done.