In reply to JMAB:
After ten minutes of obsession I reckon the way forwards is to slide in a 1 mm thick sheet of plastic under the bristles adjacent to the rock - it’ll cam the bristles some more - then pull the lot out. Something like the lid of an icecream tub with the edges removed.
I’m sorely tempted to fly up the A1(M) and try it, but I have urchins to attend to.