Thank you everyone - some great ideas, and I have Matt varnish in the garage. Taking the car apart to spray it won’t be a problem and is a good idea too. I’m wondering if I can mask off windscreen wiper sweeps on a windscreen and use the varnish to make the rest of the windscreen look like it’s speckled with road grime.
I also like the idea of the wheels being tuned at an angle - is that just a case of bending the axle at the appropriate places?
If you can get away with bending the axle then yes. I had to cut my wheels off and reset them. Dirt around the chassis and wheel arches also looks good. A tarpaulin load in the back of a flat-bed also adds interest.