Thanks Michael. I've already been playing around with slope as well as standard deviation filters and these give a really good result of where each of the features are. I guess what i'm trying to do is to map the extent of gullying across the DEM if that makes sense? I guess I can just reclass to include all slopes above a certain angle and that will do the trick.
I'll give the cut fill tool a go as well as I think i've just found a way to create a rough 'before' surface.