Put the joint of meat into a large, deep saucepan. Pour in enough cold water to cover the meat by a third, then add the star anise, cloves and peppercorns and then the bay, crushing the leaves as you go.
Peel the onion and halve it, then tuck it in with the meat. Chop the celery rib in two or three short pieces and add them to the pot.
Place the pan over a moderate heat and bring the water to the boil. Skim and discard any froth that rises to the surface, partially cover the pan with a lid, then lower the heat to a gentle simmer, the stock giving a mellow blip-blop.
Leave the ham to cook, turning it over occasionally in the quietly bubbling stock, for 90 minutes then turn off the heat.
Meanwhile, halve the apricots and discard their stones. Put the fruit in a saucepan and pour in the water. Cut two or three long strips of peel from the lemon and drop them into the water together with the juice of the lemon and the sugar.
Bring the water to the boil then lower the heat and let the apricots simmer for about 10 minutes until just soft enough to crush. Remove the pan from the heat and set aside.
Crush half of the apricots to a coarse purée using a fork or the back of a spoon, then stir in the marmalade.
Set the oven at 200C/gas mark 6. Carefully remove the meat from the cooking liquor and lower it into a roasting tin. Place half of the apricots around the joint of meat, together with a few tablespoons of the cooking liquor to keep them moist during cooking.
Spread the purée over the top of the ham (some of it will inevitably slide off into the roasting tin). Place the ham in the oven until the apricots are totally soft and the glaze is starting to brown lightly in places.
Remove the roasting tin from the oven, cover with foil and leave to rest for 10 minutes. Carve the meat thinly, then serve it with the apricots that have cooked around it.