Quoting "Bill Y." :

When I tick on Monochrome, Red shows at 100; Green and Blue are zero.

I have a feeling this is not right and that I have somehow changed the default settings. I was practising with the excellent 'Hair' tutorial when I began to suspect that something had changed and I could no longer achieve the differentiation of Greys.

I'm not sure what you expected to see but when you use the Channel Mixer in "monochrome" mode, the output is the result of taking the weighted average of the three channels The percentages set by the sliders specify the weight of each channel, so with Red=100, Green=0, and Blue=0 the result will be only the red channel of the image. If you were to change the weights to Red=21, Green=71, and Blue=8 then the result would be equivalent to what is produced by performing "Colors->Decompose: Luminosity".

Perhaps the functionality would be more intuitive if the initial default settings for the monochrome mixer where RGB=21,71,8 instead of RGB=100,0,0; but there is nothing wrong with the way the mixer functions (and the mixer retains the last values used within any session, so it is only a matter of their initial default values).