Thank you guys for all the comments. I really appreciate it and the advice is solid stuff, just the kind of thing I was looking for.
It took a couple of weeks to put this together, lots of experimenting and hunting of stock involved but it was good to work on an image this big.
Alas, the mask in the window is the best I can do for adding some driver detail. I'm just no good at painting stuff and was unable to find a stock image that I could use that would create a realistic driver. This is a point that needs more work.
Would I be right in thinking that the shadow under the car needs to be fainter, as in blurring it some more so that it sits better on the road? I also see about the brightness of the car. I hadn't noticed this until I compared the white parts of the car with the other whites in the scene. Well observed that one.
The window light was a way to add a more night time atmosphere but it's obviously too bright. Easy enough to change though as all the elements to the image are on separate layers, even to extent that there are nine layers for the headlight lights alone. I did get a bit carried away with them.
There is a bit of a shadow behind the kitty - I do love to put one of the furry fiends in my pictures if I can - but it's obviously not working too well so more work needed on that one.
I did use the G'mic filter 'Mixer rgb' which is under Colours to do the initial darking of the image, it seems to have a good effect by moving the red, green and blue brightness sliders down, but I'm going to check out 'patdavids' mute colours script and see how I fare with that. Thanks for the tip.
It means a lot to know what other folk think of my stuff, so I was well pleased when I logged in tonight. Thank you all, and now lots of work to do to bring this one up to scratch.