Bought some of the Halfords (not Humbrol) matt lacquer today and tried it on my test piece.
It looked good on the red oxide primer but on the matt black it changed the colour to more of a lighter charcoal black. In both cases it added an extra sheen to the underlying colour which is to be expected as you are applying a transparent refractory layer to the underlying matt finish.
I also tried thinned Ronseal polyurethane (solvent type) Mattcoat on the test piece using a fine brush. This made the underlying matt black glossier but had less of a gloss effect on the red oxide primer. Paul Freshney, who is an acknowledged model boat painting expert, considers that careful application of thinned Ronseal with an airbrush can give a much more graduated and controlled effect and I would certainly endorse that having seen his superb recently completed Sievers Mersey lifeboat kit.
My current conclusion, subject to anything Halfords might say, is to stick with their matt black as it comes out of the can which does give the impression of being a top coat. The matt lacquer looks a good bet for sealing the red oxide primer and gives a fairly similar sheen level to the matt black from most angles.
Colin