Preheat the oven to 200 degrees C. Wipe the ducks inside and out with a damp cloth. Slice the bread in half, lay it in a dish and pour on the port. Leave to soak for 2–3 minutes, then put two pieces of bread inside each duck, together with a sprig of thyme.
Dust the birds all over with the seasoned flour and place them on a rack in a roasting pan. Dot the breasts with the butter and roast in the oven for 50–60 minutes, basting three or four times. Alternatively, you can spit-roast the birds for 50–60 minutes, basting them regularly with melted butter as they cook.
While the birds are roasting, make the sauce. Put the port into a small saucepan, add the golden shallot and simmer on low heat for 10 minutes. Stir in the thyme jelly and, when it has melted, add the vinegar. Taste and adjust the seasoning, adding a little more vinegar, if desired – the sauce should be sweet with just a hint of sharpness. Pour the sauce into a heated gravy boat and keep warm.
Remove the ducks from the roasting pan or spit and place them on a heated serving dish. Skim all the fat from the roasting pan, or from the drip pan beneath the spit, leaving the juices behind. Spoon the juices over the birds and serve with roasted potatoes, roasted parsnips, lightly boiled leeks and brussels sprouts, if desired. Serve the sauce separately.