Thank you guys. With the exception of the fenders (Aber), this model (The old Dragon kit, number 6208) was built entirely out of the box.
The mud is a mixture of fine sand, static grass, acrylic paints for colour and polyfilla. This gloopy mixture is stippled on with an old, stiff brush and then shades of tone added with MIG pigment. The wet mix can be kept in a small airtight pot and as long as you top it up with water periodically, it seems to last for months at a time. Frank, yes, the plan is to put it in a dio - I'm just working on the secondary vehicles for the scene.
Sean