Just to clarify:
On the Met Office charts:
Red area= 2*10^2 mg/m^3 ash concentration
Black area= 2*10^3 mg/m^3 ash concentration. In other words, the concentration at the edges of the black contour is 10* that at the edges of the red area.
Around the black area is added a (non uniform) ~60nm buffer zone, to allow for shifts from one 'snapshot' to the next, six hours later. This is the 'no-fly' zone.