Sounds like slugs... for which you'll get lots of information for control... here in the U.S. there are two things that really help. At your local DIY store, buy a bag of play sand... the kind kids put in sand boxes... open the bag near your garden and pour it out and let it dry, whcih won't take long if you'll spread it out. When it's dry, pour a line around the cabbages about 3 or 4 inches wide and maybe just 1/2 inch deep. Keep it refreshed thorughout the season. The slugs hate the rough dryness of the sand and won't crawl over it...
The other method works pretty well, but needs to be refilled almost daily. Use the smallish styrofoam dish one usually uses for summer picnics... not to big. Form a small indentation in the earth near the cabbbage just deep enough into which you'll place the dish with the rim flush with the ground. Pour a goodly amount of beer into it... at least a couple of inches deep. The slugs are drawn to it, climb down the sloping side of the dish and drown in th enticing aroma of the beer.
There are commercial poisons available that smell like beer, but almost all warn about using thema round pets (or children).
Best of luck!