Sure does make sense.

It's not that much of a prob, though I must admit, with the myriad of options to be had, I must have just got lucky this time..

What you need is a Layered Shader. You need to add at least two materials to it, I guess you could use a seperate material for the cracks and a seperate material for the grunge and crap.

You want to use an image with transparency, the cracks and grunge being coloured and opaque. I assigned this image to both the colour and the transparency channels of a lambert.

You also need your standard mirror shader, blinn, whatever.

After you've got these two shaders, make a layered shader with the cracks shader on the left and the mirror shader on the right (of the Layered Shader Attributes) section as seen in the attribute editor. I guess you could use two seperate images for the cracks - 1 coloured and 1 black and white transparency-map. I haven't bothered to check if it makes that much of a difference to the finished image.

Here's what I got when I applied these settings.
I can send the scene if this isn't very clear.

Edit: Nice image btw, I guess I shouldn't ask what a nice lady like that is doing in the gents?