Alex is spot on.
I have a few on my rounds,
The gardiners black flockd brush gets them off,
But at the minute I've found the tecbuk hybrid is the best brush to get the marks off. It does it with relative easy because the bristles splay out so well it almost gives the effect of rubbing it with a microfibre.
A spray of virosol diluted 1-10 on your brush head also does it. (Bit too much of a pain at 55 feet though.)