That is typical mollusc damage. Bet it is hiding under the pot.

Looks like slug or snail to me too Owd! Turn it up and make sure that there is nothing hiding in the holes of the pot, otherwise between the sides of the compost and the pot. You could try some of that copper tape to keep them off the pot, or stand the pot on a brick inside a tray of water and see if it stops the munching.