Thankyou so much for your suggestion Chashio, it worked perfectly, with a little fiddling around.

Step by step;

1. I did the dirt layer as normal.
2. Created a snow mask channel.
3. Created the texture and bump mapped.
4. Add the layer mask and then blur the mask.
5. Repeat step 4 2-3 more times depending on how smooth you want the transition.
6. Apply a final layer mask of the snow channel to cut the edges off.

Texturing isn't perfect and the map is a little basic for now (it's nowhere near finished, just wanted to show the final product of the snow).
Click image for larger version. 

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Thanks again