Double (heavy) cream is easier to float, but whichever cream you use, the trick is to whip it or shake it to get air into the cream. Then pouring down the handle of a flat-end bar spoon is my preferred method of layering the cream over the cocktail.
I deffo know that about double cream as I float it on my black coffee in a morning, what I did NOT know is that you can do the air shake with single cream. I will give it a try, thanks Simon!