Place the potatoes in a saucepan and cover with boiling water. Cook for about 15 minutes or until just tender. Drain and leave to cool, then cut the potatoes in half.
While the potatoes are cooling, mix together the mayonnaise, yogurt and lime rind, and season with pepper. Set aside.
Pull and twist off the lobster claws and set aside. With a sharp knife, cut the body in half lengthways, from tail end through the head. Remove the meat from the body/tail shell and the claws. Chop all the meat into chunks. (The meat from the spindly legs can also be removed with tweezers, but this takes a lot of effort for the small amount of meat inside them.)
Toss the potatoes with the shallots, snow peas, grapes, watercress and lime dressing. Arrange the rocket on large plates and add the watercress and potato salad. Scatter the lobster meat on top and serve.