I'm not familiar with the Bowen Cap, but I've had good luck with a basecoat of black, followed by drybrushing with Delta Ceramcoat dark burnt umber, then Delta burnt umber, then a mix of the burnt umber and Delta Autumn Brown. I used Krylon Matte to seal it (the old Krylon Matte had a cool sheen to it - perfect for leather. Sadly, the new stuff is too flat
). For added weathering, you can scuff it slightly with sandpaper on the high spost (knees, etc.)
I used those colors / techniques on Killing Time's "Storm" kit (sorry the pic is so small):
Hope this helps!
-Phil