Having had issues with both W&N and Daler Rowney recently, I tried AK Interactive Ultra Matte.
Its fantastic!!
It comes as a very thin liquid, designed for air brushing with the addition of thinners, but can be used straight from the bottle with a brush.
2 thin coats and I get the mattest finish I have ever had. No frosting or chalky bits and although it needs a good shake, its nothing like the shake the W&N or DR needs - 15 seconds or so seems to be perfectly adequate.