Beat the milk and eggs together, strain into a jug and stir in the sultanas, vanilla, a generous grating of nutmeg and the orange zest. Chill to allow the flavours to mingle while you prepare the tart case.
Heat oven to 200C/180C fan/gas 6 with a baking tray inside. Roll the pastry out a little thinner on a lightly floured surface until large enough to line a loose-bottomed 23cm tart tin, leaving the excess pastry overhanging at the top. Chill for 20 mins. Line the pastry case with baking parchment, fill with baking beans and cook on the heated tray for 10-15 mins. Remove the beans and parchment, and bake for 5 mins more to cook the base of the tart. Turn the heat down to 180C/160C fan/ gas 4. Trim round the edge of the pastry case with a sharp knife to remove the excess.
Carefully pour the custard mixture into the tart case, evenly distributing the sultanas, then grate over some more nutmeg. Bake for 20-30 mins until the custard is set but it still has a wobble in the centre. Cool to room temperature or chill before serving.