In the end, the only place on the street where she was out of the sodium lights was in the shadow of the Chimera soft box! This meant that the flash had to be in a certain position - two metres from the subject, about twenty degrees above her eye line and at about 40 degrees from the axis of the lens. As a set of restrictions, these aren't too bad and so I set about getting light and flash readings so that I could get the shot done and get both of us into the warm again.
A straight reflective reading from the sky (through a 16-35 f2.8L lens on a Canon EOS1D MkII) was one second at f4 on 200 ISO and so I set the flash to 1/4 power (50 joules) and did a test shot. This overcooked the flash by a small amount and so I stopped down by half a stop. The test shot showed the sky a little less dramatically than I'd hoped it would be and so I went to two seconds at f4.5. From the look of the LCD screen this was just right but I played safe and shot the shutter speed with some bracketing from four seconds to one second (one stop over and one stop under).
Hand holding at shutter speeds of two or more seconds is never going to be an exact art and so I shot a lot of frames that were very similar. By getting the subject into the "shade" I had pretty much guaranteed that she would be frozen and so I could be pretty carefree with my stance and keeping the camera still. I had enough time to shoot some pictures with a 70-200 f2.8 L IS lens with image stabilisation turned on as well. These images were remarkably sharp, but at the end of the ten minutes I knew that I preferred the pictures made with the 16-35.
On the topic of white balancing, I am quite cavalier in my approach because I shoot RAW but I have noticed that this particular Chimera soft box has started to get warmer over the last few months. I have worked out that I should shoot with the white balance set to 4850 degrees if I want to save myself the bother of making the changes in Adobe Camera RAW.
Sometimes you know that you've done a good job and driving home to get warm and edit the shoot I actually felt quite excited!