Bring enough water to boil to cover the crabs. Add the crabs and boil gently for 20 minutes, then remove from the water and leave to cool. Remove the claws and legs.
Break the bodies open carefully and remove the brown meat from inside the shell. Transfer to a bowl with any juices. Remove the white meat from the claws and shell and mix with the brown.
Add the chilli, most of the chopped parsley, the lemon juice and the crushed garlic to the crab. Stir in the olive oil- you want the sauce to be quite liquid.
Cook the pasta in plenty of boiling salted water until al dente and drain well. Stir into the crab sauce, but do not reheat.
Serve sprinkled with the remaining chopped parsley and a generous amount of extra virgin olive oil.