Light the grill. Meanwhile, get started on the potato and ramsons purée.
Boil the potatoes in seawater until soft. Drain the potatoes, but keep the potato water. Smash the potatoes with a ricer in a large bowl, and mix them together with the finely chopped ramsons. Beat the potatoes with a whisk into a light purée. Stir in a little of the warm potato water and a few drops of olive oil, little by little, until the purée has a smooth consistency. Season it with sour cream or a splash of vinegar.
Prepare and clamp the langoustines in a hinged gridiron or a grill basket. Grill the langoustines for 4-5 minutes per side until the shells crack and the juices start to seep out from the heads.
Feel free to splash a little seawater onto the langoustines while grilling for a little extra saltiness.
Place all the ingredients for the crab boil in a large pot and bring it to the boil. Place the whole crabs in the pot and let it boil for about 15 minutes for small crabs or 20 minutes for the larger ones. Let the crabs cool in the crab boil in the refrigerator.
Place the langoustines and crabs on a large platter and let the guests peel and dip the shellfish in the potato and ramsons purée at the table.