Thanks for the input guys.
I've done a few dials and only even use a cotton bud and water. I was hoping that there might be a tried and tested technique for removing fingerprints specifically. Having said that, I've now taken a close look at the mark under the scope and rather than sitting on the surface as I originally thought, it looks like it's actually discoloured the surface.
I'll just give it a very gentle clean and concentrate on the movement and case! I've never worked on a UG so looking forward to getting my teeth into it.