No it's not from OCUK, I just compared it to the filter that's in the front of my Z9 case and it is much finer and that filter catches so much dust!
I think one layer should do me fine for the top and back fans. But I guess we'll see how it goes, I think I'll see if i can mesh over the spare PCI slots or blank them out with tape as someone said above I'm pretty sure a lot of dust will creep in that way.
If you search for fine insect mesh you'll be able to find it, cost about £3 for 3 meters of the stuff.