Thank you Sarkut.
My Gradient Mapping settings were:- HOT preset, Source:Luminosity, Offset:0, Wrap:Checked, Preserve Alpha:Checked
However my end result leaves visible yellow artifacts within the image, manipulating the color slider within Gradient Mapping allows me to reduce the yellow but it remains visible. If I eliminate it completely with either Gradient mapping or Color Reaper the image is unsatisfactory. See the image below.
Your result is pretty much perfect - what settings did you use in Gradient Mapping?