They're called crawfish here in the U.S. and, yes, freshly caught crawfish require purging but that's fairly simple. Our relatives in the southern U.S. are past masters at preparing Cajun crawfish...
When you catch the critters, place them in an ice chest (one used for picnics) filled with ice and a little water (crawfish need air and can't stay submerged for very long... a dead fresh crawfish is a deadly crawfish as far as eating is concerned). Once you get them home, obtain a large box of common table salt... say 26 ounce size (we prefer non-iodized or Kosher). Drain the chest of the ice and fill with cold water and sprinkle half the 26 ounce box of salt over them... stir enough to mix the salt. Wait 20 minutes, drain, rinse and do it once more (don't drain it on the lawn... kills grass). If any crawfish float to the top and don't move during the second salt bath, get rid of them.
After 20 more minutes, rinse again... now they're ready for "fixin'". In Cajun country there's a spice bag available at grocery stores called "Crab Boil"... one just throws it in a large pot with enough water to cover the crawfish and boil until they turn a bright red... have plenty of Louisiana Hot Sauce or Tobasco available... Yum!