Preheat the oven to 200 C, Gas 6. Roll out the pastry on a lightly floured work surface and use it to line a 4 cm deep, 22 cm loose bottomed flan tin. Prick the base here and there with a fork and chill for 20 minutes.
Line the pastry case with a sheet of crumpled greaseproof paper and baking beans, and bake blind for 15 minutes. Remove from the oven, discard the paper and beans, then return the pastry to the oven for five minutes longer.
Remove from the oven, brush the base of the pastry case with the unbeaten egg white, then return to the oven for another minute. Remove the pastry and reduce the oven temperature to 190C/ 375F/gas mark 5.
While the pastry case is baking, remove the langoustine meat from the shells. In a bowl, gradually mix the milk into the clotted cream until smooth.
Beat in the eggs, then stir in the tarragon, parsley, three-quarters of a teaspoon of salt and some freshly ground black pepper. Scatter the shellfish meat over the base of the pastry case, then pour over the egg mixture.
Bake the quiche for 25-30 minutes until just set and lightly browned. Remove and leave to cool slightly before serving.