I can thoroughly recommend a product called Ant Stop, which is bait sold in a little round black covered disk container, which you can buy from good hardware shops, Robert Dyas and other places which contains bait. . it works for us every time. If you place it close to where you find the ants are making a trail, they will take it back to the nest and kill them within 24 hours. At first you will find more and more ants appearing as they can't resist the stuff, but it really does work quickly. You use a white powder but it's terribly messy and gets washed away by rain.