So it seems having cork under the track causes problems with my 'mix wall paper paste with the ballast' approach. Basically the paste flakes won't stick to the cork, and rather than soaking into the baseboard (always chipboard back in the day) it just sits there and expands if the ballast is re-wetted, say for tinting with a wash. . So for everything else (so far, anyway) I've reverted to placing the ballast dry and then misting on a 20% PVA solution with a sprayer - from a safe distance so the ballast doesn't get moved around. When it's damp a bit more aggressive spaying makes sure it get nice and soggy. Initially I topped it up with dilute PVA from a dropper, but that turns out to be over kill. Having done some replacement of the original for a test piece the difference is all too obvious . I've also manged to omit the FPL rod on the double slip. Out with the chisel....

Some testing shows that the paste problem might be the ballast itself. I'm using Woodland Scenics which it appears is made from shells, but with some old real stone ballast the method works ok.