Preheat the oven to 180C/350F/Gas 4. Truss the duck and lay the square of back fat over the breast. Place the bird in a roasting tin and put it in the oven. After 10 minutes, remove the fat to allow the breast to brown.
Cook for a further 40 minutes, basting the bird from time to time, then remove from the oven and let it rest in a warm place.
In a saucepan, melt half the butter and gently cook the shallots with the thyme for 10 minutes. Add the vinegar and let it bubble for 30 seconds then pour in the port, wine and cassis. Reduce to a thin syrup then add the blackcurrant juice and simmer for five minutes.
Add the blackcurrants, warm through gently then whisk in the remaining butter and remove the pan from the heat.
Carve the rested duck, arrange slices on warm plates then spoon the sauce around the duck. Serve with game chips and watercress.