Remove the duck’s giblets and wash them. Cut off any greenish parts from the livers and discard, then roughly chop the rest. Quarter the crops (the gullet), slicing any very red flesh away from the white sinew, which should be discarded. Put all bits and pieces into a large roasting tin.
Cut off the parson’s nose from each duck and add to the tin. Remove the wings: hold and pull on a wing, turning the skin back so you can see inside to where the wing is attached to the shoulder bone. Cut between the bones to release the wing, then cut around it from the skin side to remove it. Repeat with all wings and add them to the tin.
Turn one of the ducks on to its breast. Cut down each side of the backbone, using a very sharp, sturdy kitchen knife and/or game scissors. Remove the backbone, then turn the duck the other way up and cut down each side of the breastbone, keeping the blade close to the bone so you don’t cut away any breast meat. Put all the bones into the tin and repeat with the other ducks.
Remove any lumps of fat from inside the duck and set aside. Cut off any flappy bits of skin and discard. Wash the duck halves then pat very dry. Prick the skin all over, especially the thick, fatty areas, then sprinkle with fine sea salt.
Preheat the oven to 230C/Gas 8/fan oven 210C. Arrange the duck halves skin-side up on a rack set inside a large roasting tin. Roast for 11⁄4-11⁄2 hours, or until the skin is crisp and dark golden. You will need to drain off the fat that accumulates once or twice: pour it straight into the tin if you’re going to roast potatoes, otherwise into a heatproof bowl.
Leave the duck to rest for 10-15 minutes, serve with the gravy and veg (see 'Goes well with' recipes, right).
Up to 48 hours ahead, prepare the ducks up to the end of step 3. Up to 12 hours ahead, complete step 4, refrigerate, but do NOT salt.