Oops sorry, still a little tired hahaa. I was right clicking over the object and not the material. When I right click the material I get the option to "assign material to selection", but no "assign material to viewport selection"




Update:

I can assign another material to the object, for example I right click on the body blinn and assign to the selected object, the selected thrusters turn the right colour. However if I try again but with the exhaust blinn, it just goes back to grey?

And looking at the Common Material Properties for the exhaust blinn, the "colour" slider is subdued and cannot be adjusted up/down. Is this because it has a fade affect given? Next to the slider, where others have a black and white checkered square, this one has a small square box with a triangle.