OK, so I've been giving the weathering a try. I wanted to do exhaust stains, chipping and a pin wash and this came out all right, not great and down right terrible, respectively.
First I added the exhaust marks on the underside...
...and then attempted a pin wash
I just found it impossible to get it precise enough - I applied the wash on top of a gloss coat and tried ground up chalk pastels and watercolours, even with some detergent in to break the surface tension, but the mix would just not flow as it does on the youtube tutorials! This is the reason it looks so messy - cleaning up was a nightmare. I'll keep trying, but any tips would be greatly appreciated.
On the topside, I did exhaust stains...
...and then attempted some chipping.
I didn't attempt a wash on top because my previous attempt looked so poor. I had been pleased with the build this far, but the weathering lets it down, sadly. Still, learning all the time...