Thought i'd copy Dave Fisher in AFM and use permanent markers for the mottling.
Trouble there was that my local art shop on has black.
Tried that and it were 'orrible - FAR too dark, and doubtful if it'd dry.
Instead am going this way - using 2 shades of dark grey pencil, 3B and 5B.
Starting with the lighter of the two, mapping out where i want the mottling to go, varying full blotches with circles.
Then i'm filling in the centre of the bunches with the darker grey.
Going quite well at the mo'