Trying for the High Key look - only have the on-camera flash so using fluorescent lights to light background and camera's flash from front. But getting purple cast/highlights. Can anyone advise why, so can take without getting this colour?

Yeah, Chromatic aberration is the most likely cause. GO here to read about it: http://en.wikipedia.org/wiki/Purple_fringing

An easy way to be able to use this photo, is simply to turn it into B&W.

Also, if you're using Photoshop, you can add a Saturation layer and play with the blue, red, and magenta sliders until the photo looks right to you.

Thanks for the help - read the link and tried reshooting. Find start getting the purple (Chromatic aberration) when aperture gets above f-4. Always finding learning something new with these challenges.