How do I get my offset models to be at 0, 0, 0? Apologies, I'm relatively new to blender and have inherited this model. I have tried Alt+G and setting the origin points, but the model itself is still off-point.  This means when attempting to rotate, it's axis is about 0, 0, 0 rather than being about the centre of the image.

How do I get the base of the model to sit at 0, 0, 0?
Switch to 'Edit mode' (Tab key).
Press A key. This will select all the verts in the model.
In Orthographic view (Numbad 5) move the selected group of verts over the orange dot (the origin).
Repeat this step in orthographic view for X, Y and Z.
Back to Object mode, select the model, choose Object> Apply> All Transforms. This just resets any scale or transform issues.