We had this problem and lost a large piece of the lawn due to not tackling the problem staright away. The next time it happened in another spot we called in 'Green Thumb' and they applied a chemical treatment and that 'stopped them in their tracks'.

Apparently there is a natural remedy by Nemasys 'Chafer Grub killer' which contains nematodes roundworm like things that are parasitic and get rid of the pesky grubs.