Ian, I've tried probably every type of matt varnish imaginable, both enamel, acrylic and the old polymer stuff, and nearly always had at least one problem somewhere with all of them! The usual stuff - yellowing after a time, drying gloss, or semi-matt, drying too matt with a white deposit etc etc. One I had been using was an artists clear acrylic matt, in a large plastic bottle instead of those piddling small things, which worked very well, and probably still does. However, I left it standing for months, and it is so thick I just can't be ar**d stirring/shaking it for so long!
What I use now is any acrylic matt varnish, mixed with Johnson's Klear, then add a touch of Tamiya Flat Base, until the finish is right, established by trial and error. Too much Flat Base, and it will all turn white when dry! However, it doesn't take long to figure out the proportions, and I use it for dead matt, like the early war 'sooty' black camouflage, 'Night'; matt for 'normal' matt, a very slightly semi-matt, for RAF WW2 camouflage colours,such as Dark Green and Dark Earth, and the sheen of Luftwaffe finishes, which is more between a semi-matt and a gloss.
One of the better acrylics is from Games Workshop, which works very well out of the bottle, but, like most, doesn't dry dead matt - hence the use of the Flat Base. The Humbrol acrylic matt varnish is good also, but in ridiculously small tubs. Whichever you decide on, it MUST be stirred, and stirred and stirred, to mix all the matting agent with the 'carrier'. If not, it'll either dry 'not matt', or leave that white look on the surface.
Cheers,
Terry.