Thanks for you reply! I found it very helpful!
You are correct, that was one of the difficulties of image-capture as I want to get a really high quality image set but I can't walk inside my study area (~10m x 2m) as that would disrupt the topography and lead to differences caused by trampling, rather than erosion. It would make image capture a lot easier if I didn't need markers above the gully too. I have calibrated scale bars from Cultural Heritage Imaging with coded targets on them. They're calibrated to 1/10th of a millimetre so they're pretty accurate.
I've just done a test run using some steps on my university campus, and I assigned co-ords of 0,0,0 (x,y,z) to the bottom left target - I'm hoping that, with that co-ordinate, and the accurate scale bars, my DEM model will produce some accurate measurements.