For the celeriac purée, place the celeriac, milk and the salt into a heavy-based pan and bring to the boil. Reduce the heat and simmer gently for 15-20 minutes, or until the celeriac is tender.
Drain the celeriac, reserving the milk. Leave in the colander for 5 minutes to drive off any excess steam, then purée in a food processor or blender until smooth, using a little of the milk to help get the right consistency.
Heat a frying pan until hot, add the butter and cook until it is a light nut-brown colour, then remove from the heat and stir into the celeriac purée with the lemon juice. Cover and leave to one side.
For the blackberry sauce place the blackberries in a bowl and sprinkle over the clove, juniper berries, bay leaf, thyme and sugar and set aside.
Melt the butter in a small pan and fry the shallot and mushrooms over a medium heat for 5 minutes, or until softened and light golden-brown. Stir in the port and wine and carefully set alight (CAUTION: stand well away from the flames), then add the blackberries and a pinch of salt.
Gently simmer the sauce for 10 minutes, then purée in a blender or food processor. Strain the sauce through a fine sieve and set aside.
For the duck, preheat the oven to 180C/350F/Gas 4.
In a large frying pan on a medium heat, fry the ducks on their legs in the duck fat for 7 minutes each leg, or until pale golden-brown (this is to give them a head start as they need more cooking than the breasts).
Carefully turn the ducks onto their breast-side, and fry for 1 minute until golden-brown all over. Transfer to an oven tray and roast for 10 minutes for medium-rare or 13 minutes for medium, or until the ducks are golden brown all over and the flesh is still lightly pink.
Remove from the oven and leave to rest breast-side down. This is so that the juices will collect around the breasts, baste them and keep them moist.
To serve, carve the ducks onto a large plate, serve the celeriac purée in a small pot on the side and spoon the sauce over the meat. Garnish with your choice of fresh blackberries, caramelised apples and celeriac crisps.