What I am trying to do is create a hole in the layer mask where the de-saturation is. Say I de-saturate a black coat so it becomes grey. I then add a red colour layer under the mask so that the red will colourise the coat – with the right blending mode.
Not really Graham. You cannot "desaturate" black. (nor white, nor any shade of grey) You can only "desaturate" colour.
If you could explain in more detail (perhaps posting and example too) exactly what it is you are trying to achieve, we could offer more specific solutions.