Funnily enough, there isn't any cork under the track. Just a case of taking my time and going over the area twice. I painted PVA on, even between the sleepers, poured the ballast, tamped it with the back of a teaspoon and used an eye dropper for the PVA/water/washing up liquid mix. I don't know if I was just lucky but it didn't clump together at all.
The ballast is Green Scene light grey for N gauge.