Torch-light inspections of damaged plants on mild nights should reveal caterpillars, these can be removed by hand.
If infestations are too heavy for hand picking, control can be achieved by spraying at dusk with deltamethrin (e.g. Bayer Sprayday Greenfly Killer), lambda cyhalothrin (e.g. Westland Resolva Bug Killer) or 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)