They are vine weevil grubs. You have to inspect each corm and kill the rotters (the robins love them). I have suffered the same problem many a time (well not personally). They don't like living in non peat based compost apparently.

Had this trouble today with fuchsias. Shook all the compost onto a tray, picked out the grubs I could see and put them out for the birds and washed the rest of the compost off under the outside tap. (Which dislodged several more I hadn't found before) Good idea to do this early in the season (like now) while the grubs are still small