Thanks man. I've been messing around with it a bit now and can't get the look I'm looking for. This is tougher than I thought!

I'm going to try the third pass and set conditions to cloudy when viewing in just a bit.
Had a look again, sorry, 3 full brightness adjustments(-150 a time) takes it very dark but leaves some contrast in the alpha. The 4th makes it full black, which upon looking again is actually alright in game, had forgotten to turn off f.lux
I think i'm settled with using 3 brightness adjustments, now playing with shades of black/charcoal for the base paint. This is what I think will help you get the look(or closer to at least) you're going for.
When choosing your base colour, open the colour picker and try R,G,B 39,39,39 (H,S,B 0,0,15) (dragging to this on the pallette is easy for selecting colours, obviously you can click in each field and type the number but you can also click each field and scroll which is my preference when fine tuning a colour to use if I don't require a specific one). This was just one colour I tried and looked quite good, but try some other shades, maybe a bit darker if you want a really black car, but try and avoid 0,0,0.
Also, I tried overcast at loch drummond and it's only very light clouds I get
