The mud/dust/dirt on the greatcoat was done by mixing a tiny bit of talcum powder with a Humbrol enamel... unfortunately I did this about 6 months ago and at that time wasn't keeping a log on my various builds, so I can't remeber the exact colour, although Natural Wood or Light Earth seems to ring a bell...
I then stippled it on the areas, and built up the layers where necessary... this also gives a feathered look on the outer areas of the muddied item...
The groundwork was built up using Polyfiller and various grades of railway stones. Once the mixture was dry I base coated the area with Humbrol Light Earth. Thereafter I used several washes of oil Burnt Umber and Humbrol Matt Black and Humbrol Clear Poly in varying degrees until it dried with the effect I was looking for. The Clear Poly was put in so that here and there it would dry with a quasi wet look... The ground work was largely hit-and-miss... pure luck as it was the first time I'd tried groundwork...
Thanks for the welcome... I guess I should post an intro some time...