I dragged and dropped the new milk chocolate shader and enabled displacement and the when the fluid moves the displacement seems to stay in the same place - it doesn't flow down with the fluid but rather stays relative to where it was initially. Dumb error on my part?

You need to enable the rest option in the Fluid Object DOP and the Dual Rest fields in the FLIP Fluid Solver DOP.

Cheers for that, makes sense - I know about that in the material shop - but didn't think you'd find it outside ops: