Roll out the pastry on a lightly floured work surface and use to line a 23 cm (9 inch) tart tin, allowing a little overhang. Prick the base with a fork and line with baking paper. Place a 20 cm (8 inch) cake tin in the case, then fill with baking beans. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until the sides are set. Remove the inner tin and paper. Return to the oven and cook for a further 10–15 minutes until golden and crisp. Press down any risen bits, then leave to cool. Trim any excess pastry.
Place the milk, cream and lemon rind in a saucepan and scrape in the seeds from the vanilla pod. Simmer for 5 minutes. Beat the eggs, egg yolks and sugar in a bowl until pale and thickened. Stir in a spoonful of the hot cream, then pour the egg mixture into the pan and cook over a low heat for 5–10 minutes, stirring frequently, until it coats the back of a spoon. Leave to cool slightly.
Strain the mixture through a sieve and pour into the pastry case. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes until firm with just a slight wobble in the centre. Leave to cool.
Stir together the cherry conserve and liqueur, if using. Drizzle over the custard tart to serve.