My sprouts were suddenly attacked by mass colonies of grey cabbage aphids, to add to the squadrons of resident whitefly.
I stripped off all leaves except the outer leaves of the top "cabbage", and removed the cabbage centre, then used a hose jet to thoroughly wash down the plants, followed by a feed with a pure nitrogen fertiliser. They now look clean and decent, and the sprouts are starting to grow again.
The odd colony of aphids, and any caterpillars get a quick squib with a pyrethrum spray.