I have recently moved and constantly coming up against new problems in new house 
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So I have a small extension to the ground floor that juts out into the garden. One wall of this extension is about 10 inches from neighbour's fence. There is rubble and mud in this gap covering over the damp proof course. DH has dug out the mud right next to the wall to below the damp proof, but there is now a muddy bank holding up neighbour's fence sloping down to the wall. My question is what should I do now? What can I put in the gap that will stop damp but prevent the mud sliding back down to the wall?