Jacs,
I'd mix the talc with some clear. You'll have to experiment with the mix to achieve the flatness you desire. You'll also have to additionally reduce the mixture to provide the sprayability you seek.
In the auto paint lines I worked with the mixture was talc and a binder. This was added to the clear or the color to achieve the flatness.
All of the colors were formulas that we would follow to mix. If the mix does clog the tip, just run some reducer through the airbrush and add more reducer to the flattener.
What is this flat paint for?

Good Luck
Pat

P.S. I went to this website and Humbrol seems to have a variety of matt colors.

http://www.londonbridgetoys.com/humbrol.htm


This website includes a color chart.

http://modelsforsale.com/humbrolpaints.aspx