Just to show what clay can do - before today, I last clayed my paintwork about 12 months ago, and this morning I used the Bilt Hamber stuff, having washed the car beforehand.
I wash my car at least every other week, usually weekly, and it's pretty much always got at least 2 coats of wax or similar on it. Basically, I keep it pretty clean.
And here, next to an unused bit of clay for comparison, is the result of claying JUST THE FRONT PASSENGER DOOR.
Phil - if it is under the clearcoat I'm sorry to hear that. Could be worth trying Scratch X, and even if it won't fully remove it, at least if it shows some improvement you know that a machine polish could well sort it.