What a mess! Although I've carefully tried out patches of various ballasts in off-scene parts of the layout, I made the mistake of using one batch untested. Greenscene's 2mm ash ballast was a safe bet, I thought, on the grounds that a couple of sidings didn't really matter and the ultimate in appearance wouldn't be required. Unfortunately the stuff is too dreadful for words - it's exceptionally coarse (about the size for 7mm) and has a host of horrible brown bits in it that look like they haven't been coloured properly.
The whole thing had the look of dyed sawdust on a 1960s 'Railway of the Month' so I've started to scrape it off, as you can see by the various holes.