At last - some progress....
Having had a busy few months at work, and waiting in vain for the ballasting fairies to visit - I have finally decided to get on with it.
The idea is to fill the largest areas with haphazard stonework, and then infill with modelling clay, stippled to represent (hopefully) ash ballast, to rail level.
Rail level will actually be kept clear of, as I want to make sure the rather short waggons have the best chance possible. Anyway, maybe my imaginary line did that
I am trying out some awesome now clay - my wife bought it for our 4 year old to use - it is very light, appears not to be water based, and is fairly adhesive. Imagine white blu-tack, only far less dense. It is called Crayola Air Dry Model Magic. (hopefully I am ok saying on here). soooooo much better than the usual air drying clay - apart from anything else, it isn't clay passed, so it doesn't get everywhere. It doesn't seem to shrink or crack, and is incredibly light when dry.