If they are halfmoon shaped, they are likely to be wineweevil grubs (which eat the roots of plants) These grow into horrible brown beetle type creatures that munch on the leaves and flowers. I use Provado and find it works.

I just looked this up and Titchmarsh recommends keeping the surface of the compost free of debris which hides the adult females. Ant to use a thick mulch to stop her laying eggs in the compost. Also use insecticide granules which is what Dylandog has just said.

Certainly sounds like vine weevil grubs, I'd empty the container, clean thoroughly and bin (not compost) the soil you remove. Stand the cleaned container on feet or small rocks or bricks to raise it up off the floor, cover drainage hole with something like a piece of 'J' cloth fill with new compost and then replant your plants, having washed every last particle of soil off them. Even Provado wont kill the eggs only the grubs so you will still have a problem. Oh and watch out for the adults chewing on the leaves remove and squash or stamp on - they are tough little blighters.

Mazziestar Alan's advice works to prevent the vine weevils, sadly it does not do a thing once you have got them.