Have a look at non-adhesive shelf liner. You can make it whatever size you like, it comes in matte black (as well as matte white, taupe beige, and a few other colors) and the non-slip surface on one or both sides is great for putting it on top of a functional flat surface like a desk. I use it all the time on everything from tables to shelves.
I like the ones made by Duckbrand. Con-tact brand and generic options will likely work, too. Size options are organized by width of the roll, 12" or 20" (30.5 centimeters or 51 centimeters). The rolls come in various lengths from 4 feet all the way up to 22 feet. I cut it to make whatever size I want with a razor blade/utility knife, and then I carefully line up the edges and tape them with a strong tape like duct tape, preferably in a matching color. Flip it over and the seam is nearly invisible, especially in the mesh-surfaced two-sided grip version called SelectGrip. The non-mesh one-sided grip version called SolidGrip is trickier to align seams, but with a little practice the seams are almost invisible, too.
You probably should decide if you want a smooth desktop surface where things will slide around, or if you want a supercool anti-slip desktop surface where your pens and other light objects will have a bit more friction-adherence.
Imagine a playful cat on your desk. While the cat can certainly enjoy herself batting your pens and pencils around on the two-sided grip mesh surface (SelectGrip), she would much prefer playing with your writing implements if your desk were covered in the one-sided grip surface (SolidGrip). As a cat owner, I like to have things on my tables remain on my tables instead of ending up on the floor to become cat toys. I tend to use the double-sided SelectGrip for most flat surfaces.
Hope this helps and best of luck with your desk!
P.S. The double-sided-grip mesh SelectGrip is completely matte. The single-sided-grip SolidGrip has a bit of a sheen to it. You can see sheen details at duckbrand'com and I believe they can ship to you across the pond.