Well, chaps. Done some tests with WD40, white spirit, SRP, AG engine cleaner (acidic) and easily the most effective substance for removing this stuff is .... alloy wheel cleaner
, and second was the engine cleaner
... yeah, I know what you're thinking, acidic cleaner and all. Just a small patch about 2" square for each test. I took some photos, but it's almost impossible to see anything on them.
I did a further test on a larger area with some Turtle Wax Nano wheel cleaner, left it on for 1 minute and gently agitating it with a soft cloth - I could feel the roughness disappearing, then rinsed off with plenty of water. It removed as far as I can tell 100% of the brake dust, and doesn't seem to have affected the clear coat, smooth and shiny.
Followed the area up with SRP and Meguiars Tech Wax and the finish looks clean, shiny and pretty good.
As this is a one off, I feel it would be ok to proceed with this, and due to a lot of stonechip damage and some scratching and swirling, I was contemplating a full respray, so I've nothing to lose, 'til I really bite the bullet and get it sprayed.
However, if anyone has any serious misgivings about this, i.e. don't do this 'cos your paint will dissolve within a week!, then please shout up now.