Thanks Dan.
On the next figure I'll take more pics as I go along, but hopefully I can explain the process.
1) Base coat the figure in dark grey. I used Humbrol matt enamel 27
2) next day, once he's dry, paint some light grey (Humbrol 147) onto a dimpled surface. I use the plastic boards from the bottom of a shopping bag! You'll see the painted patch in the center-left of this picture:

It should be painted on very lightly so it's not pooling in the dimples.
3) Using a square of kitchen towel or other textured material wrapped around your finger pick up some of the paint from the board and dab on to an area of the figure. Make sure not to slide the towel across either the paint or the figure. Dab it straight to the surface. The combination of dimples on the board and the towel gives you a completely random patern.
4) Repeat step 3 until the figure is covered. Always use a clean area of the towel each time. You'll also need to refresh the paint patch on the board, but don't let it get too heavy.
5) Once the figures use a pin to place some paint spots and dashes in the spots you couldn't reach so there are no large areas of plain dark grey
6) Optional: apply a light light-grey wash. I didn't do it on this guy but did to this other figure:

You can decide which you prefer.
7) Detail the remaining portions: face, boots, hands, etc. I did the velcro strips on the front & back with a 50/50 mix of dark grey and buff
Hope this helps
Sean