I went on CGTextures and got some great splat effects and used these to create some create brushes. Adding in all the flecks and speckles of mud has given a much more realistic look.
You can really tell I've been playing Dirt 2 for the past few months. Watching all those rally cars and trucks drive through thick and heavy mud definitely influenced this pieces outcome.
I'll get to rendering out a full 360 spin next. Will take my poor PC a few days as there's so many textures. I must also look into upgrading my graphics card. It has been pushed to it's limits these past few weeks.