Is it possible in NifSkope to modify an existing creature mesh so that it uses two textures, one that is overlayed on the other? (sorry, don't know the modelling terminology, so am using my limited photoshop terminology )

What I would like to do is make some creatures that use a vanilla texture (so compatible with any texture pack), and then have a texture that sits on top to simulate detail and shadow?

Any help much appreciated.

I think you can accomplish this using a detail map, which iirc is a type of decal.

Yep. In the Nifskope right-click on the NiTexturingProperty of the shape you wanna add the texture to > texture > add detail map.

Cool. Does it also allow for "gradient alphas" (so colour mixing can be done), or is it purely detail texture or base texture? Hopefully the first, as a 1bit detail would be pretty ugly.

I don't fully understand everything in the question but no, it does not completely cover the texture bellow so yeah, the colours or the original texture and detail map will mix a bit.