Thank you very much everyone.

I've been pulling my hair out over this today, and thanks to your help I now realise what I was doing wrong.

I only had the 3D viewport open when I was using the Cplane tool, and as a result I did not see any change when using the tool.

With all viewports open I immediately seen the object rotate and the face become normal to the view. Exported the applicable faces and imported into another program, and checked dimensions. Everything perfect.

I'm a very happy camper,

Keith