Combine the orange-flavored liqueur, lemon juice and sugar in a large bowl (I use the bowl of my freestanding mixer) and stir to dissolve the sugar, or as good as. Slowly stir in the cream then get whisking. As I said, I use my freestanding mixer to this, but if you haven't got one, don't worry - but I would then advise a handheld electric mixer. This takes ages to thicken and doing it by hand will drive you demented with tedium and impatience. Or it would me.
When the cream's fairly thick, but still not thick enough to hold its shape, dribble with the flower waters and then keep whisking until you have a cream mixture that's light and airy but able to form soft peaks. I always think of syllabub as occupying some notional territory between solid and liquid; you're aiming, as you whisk, for what Jane Grigson called "bulky whiteness." Whatever: better slightly too runny than slightly too thick, so proceed carefully, but don't get anxious about it.
Spoon the syllabub in airy dollops into small glasses, letting the mixture billow up above the rim of the glass, and scatter finely chopped pistachios on top. In my How to Eat cookbook, there's a recipe for pistachio crescents that would be fabulous dunked into and eaten with this. But only if you feel like it: the cool, fool-like smoothness of this is perfect as it is.