Using an electric mixer, whip the cream cheese until it is light and fluffy. Add in the capers, shallot, dill and lemon juice and mix well. Using a rubber spatula, fold in the smoked salmon pieces.
Divide the cream cheese mixture onto five slices of the bread and spread it evenly to the sides. Remove any large stems from the water cress, and top the cream mixture with a thin layer of watercress.Top each sandwich with the remaining slices of bread. Cut the crusts from the sandwiches and cut each sandwich into 3 rectangles. Cover well and refrigerate until needed.