Torch-light inspections of damaged plants on mild nights should reveal caterpillars; these can be removed by hand. The caterpillars can be placed on wild plants such as nettle and bramble to continue their life cycle.
If infestations are too heavy for hand picking, control can be achieved by spraying at dusk with synthetic pesticides such as deltamethrin (e.g. Bayer Sprayday Greenfly Killer), lambda cyhalothrin (e.g. Westland Resolva Bug Killer) or the organic pesticide pyrethrum (e.g. Py Spray Garden Insect Killer). The older caterpillars are more tolerant of pesticides than young larvae. Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)