Wash leeks thoroughly, but do not dry. Melt butter in heavy large skillet over low heat. Add leeks, cover and let sweat until tender and translucent, stirring occasionally, about 10 to 15 minutes. Remove from heat and set aside.
Add Fish Fumet and sherry to skillet and heat just until liquid trembles. Add scallops and poach 2 minutes. Remove scallops using slotted spoon and keep warm. Increase heat and boil liquid until reduced by half, scraping up any bits clinging to bottom of pan. Add cream and continue boiling until thickened. Return leeks to skillet and heat through. Season to taste with salt and pepper. Divide leeks and sauce among 4 heated serving plates and arrange scallops on top. Serve immediately.