Place the salmon in a non-metallic bowl. Pour over the lemon juice and toss the salmon in it until all the pieces are coated. Cover and leave in the refrigerator to marinate for 3 hours.
Drain off the excess lemon juice and discard. Mix the salmon with the cucumber, capers, chopped tarragon and mayonnaise, season with salt and pepper and serve on silver or clear spoons topped with dill sprigs, if liked.