Heat oven to 180C/fan 160C/gas 4. Roll out the gluten free pastry and use to line 8 x 10cm fluted loose-bottomed tart cases, leaving a little pastry overhanging.
Prick the cases with a fork and bake blind for 20 mins (removing the paper and beans after 15 mins) until pale golden brown. Set aside to cool. When cool, use a serrated knife to neatly level the pastry.
Meanwhile, put the zest, juice, lactose free butter and 225g sugar into a large heavy-based pan. Heat gently until the sugar dissolves, then remove the pan from the heat.
Beat the whole eggs and yolks together, then stir into the pan. Stir over a low heat for 5-10 mins until thickened. Sieve into a bowl, then spoon into the cases. Any leftover curd will keep in the fridge for up to 3 days.
Whisk the egg whites with 50g sugar in a grease-free bowl until stiff peaks form. Whisk in the vinegar, then gradually whisk in the remaining sugar.
Put a good dollop of meringue on top of each tart, then dip a palette knife in hot water and make little peaks on each. Place on a baking sheet and bake for 15 mins or until just browned on top. Allow to cool, then serve.