It's as though there is no 'bite' to the primer whatsoever. I've since received some advice as to where I might have gone wrong (applying it too thickly instead of thinner coats, not letting it cure for a long enough period) so I will reserve judgement and experiment further on an test piece.
In the meantime I washed it all off using IPA (Iso Propyl Alcohol) and will go back to the Games Workshop rattle cans for now. I have to use white as the areas for the national markings will be masked off prior to painting as dark paints will only show through the decals.
Also the primer had to be removed as I'm not sure what reaction would occur between the Vallejo and putting the Games Workshop stuff over the top (probably a bad one knowing my luck).
Now I can get on with applying masks for where the national markings will be and painting the thing. Finally!