After brushing off all the loose sugar (what a mess!) I then applied a thick lathering of wet glue all over the top of the two boards. I actually put too much water, and it was quiet wet, so I'm hoping it will dry okay.
After brushing the wet glue all over the surface, I then scattered small 'slate' gravel all over the board, and made it thicker in streaks in some areas. Then I glued some bigger 'slate' stone pieces (still pretty small things) making sure to pick ones that were wide and long, but relatively flat, so they wouldn't cause problems when sitting sceney on top of the board.
The wet glue had a good side, in that it allowed the gravel stuff to half sink into the board, which should make it set better into the surface, but also looks in some places like it is beneath the snow/ice.
The light gray colour of the foam board has given a kind of shadowy tint to the thinner/clearer parts of the board, letting the thicker sugar areas stand out a bit more, which I quite like.
Using a hairdrier, I managed to get a part of the board relatively dry, and sat stuff on it to take some photos, to get an idea of what the finished board will be like to use.
and here's what the two boards look like at the moment (though they are still drying)