Here is the tree as glued in with Gorilla PVA before I started to blend it in.
The 'gunk' mix, it lives in an old tester pot that formed it's basis some time ago (this mix is towards the end if it's life despite a recent revival). Application will be by dental tool.
Applying the mix and getting some shape to the top of the roots.
Applying acrylic paint, whilst the plaster mix was still wet underneath.
That's basically it - the colour I mixed is not that close a mix to the Hornby colour and I may need to touch in some visible bits of the original colour at the top of the tree to blend it all together.