The flash was off to my left at about thirty degrees from the axis of the lens. Whilst there wasn't any real breeze, there were dozens of people about and, like most touris magnets around the world, the area is well known for bag snatchers and pick pockets. Because of this I kept the kit with an arms length and shot one handed with my left hand firmly grasping the light stand. This meant that moving the flash wasn't going to be too easy and the first test frame that I fired on 50 joules was about half a stop over exposed.
A quick stop dwn to f6.7 put the flash exposure right and so the ambient exposure (controlled by a combination of aperture and shutter speed) needed to be adjusted to 1/45th at f6.7. The ambient light on the subject was enough to focus with, but not enough to affect the overall exposure.
After shooting this "obvious" shot we moved next to the edge of the pedestrianised area where i used a "bungee rubber" (strong elsatic with a hook on either end) to lash the flash set up to a railing. I shot some frames on a long lens with the squashed perspective but my first idea (as is often the case) won the day.