Many thanks jayfella - anything would be helpful.
Though I’ve just also discovered this thread:
Where t0neg0d gives a solution that (after removing a bug in the posted shader) might do just what I need, by the looks of it. I just hope it works with transparency and particles…
I understand about learning shaders myself but I hoped this was just a bog-standard fog feature (which people have been asking about for years). Could t0neg0d’s solution could be incorporated in the core code if it does the job? Would save having to hunt through forums.