Of course, Gareth. With pleasure
You see
Click to see full-sized image
Chipped paint is an additional layer, which is peeling off. It should be visible that the background is "behind" it - and it's easiest to achieve with highlighting and shading.
Check the picture I linked: the paint casts shadows, and the shadow usually is on the opposite side to the light. As light usually comes from top (eg. from the sky), the shadows are most obviously expected to be under the chipped paint. I think the photo shows it.
On the other hand miniature painting utilizes highlighting, too. So you can use it to definte the opposite edge of the patches of paint - the ones where the light falls.
So you should paint a white (or pale) line on the upper edges of chipped paint, and black line below the chipped paint:
Click to see full-sized image
Check the shield out.
It's not the only right way to do it, but the most obvious and intuitive.
If you place your shadows and highlights the other way, it can make sense if your light is coming from below the mini. But when there's no light source presented, most people will automatically assume it's natural light, which means it comes from above. Otherwise it tricks the eye and can look wrong.
Your pics seem to show you did it different... I can't quite tell how, but it doesn't look credible this way.
Click to see full-sized image
Click to see full-sized image
The pics are described in Polish, but with the explanation above they should be pretty easy to understand...
As for the dust/dirt - Demi is right. Have you tried pigments or ground pastels?