To make the glaze, place thyme, maple syrup, vinegar, salt and the orange zest and juice in a large saucepan and cook for 12 minutes or until reduced by one third. Keep an eye on mixture to ensure that it does not overflow. Stir in the mustard and stand until required.
To prepare the ham, preheat the oven to 160°c. Grease a large, shallow roasting pan. Line with greased foil, then baking paper. Sit the ham on a wire rack over roasting tray. Use a small, sharp knife to cut around ham shank. Add 2 cups water to tray and bake for 20 minutes to help separate the skin from the meat. Remove from oven, then, when cool enough to handle, carefully run knife under the skin, around the edge of the ham. Gently lift off the skin in 1 piece by running your fingers between the rind and the fat. Thinly slice oranges and use the base of the thyme stems to pin orange slices over ham, overlapping slightly. Pour over glaze.
Increase oven to 190°c. Bake ham, basting with glaze every 20 minutes, for a further 1 hour 30 minutes or until ham and orange slices are golden and caramelised. If ham should begin to burn or colour too much, cover loosely with foil. Serve with roasted potatoes.