Hi James
I have used Craftex products successfully on varnish before. They have a solvent specifically for these type of stains, and i would put some on a clean white towel or thick tissue then blot the stain out. You will see the colour transfer to the towel or tissue.
When you have blotted out all of the stain, extraction clean over the stain or the whole carpet if required. Prochem also do a good product for this type of stain as well.
Dave