Do you get the expected results if you correct the reversed faces on the top and bottom of the cabinet? There should be no exposed blue back faces in your 3D model.
I would also suggest making the back panel a components (it’s all loose geometry at the moment), turning off Length Snapping in the model units, and increase Display Precision so you can see if you have any small errors creeping into the sizes of the objects.
FWIW, I cleaned up your model, fixing the reversed faces, making the components solids, and purging unused stuff that was bloating the file. 500 cabinet.skp (249.8 KB)
Thank you everyone so much! Wish I had asked the question sooner to avoid wasting so many hours scratching my head!
Both methods suggested have worked well and I understand that I made a drawing error with faces.
This is a very handy thing to have learnt because when a panel has multiple operations on 2 faces I can select the one I want to be used when its cut on the CNC in sheet form and then change it for the single second operation.