Preheat the oven to Gas Mark 4, 180ºC, fan 160ºC.
Put the flour and sugar into the bowl of a food processor and chop the butter into the flour in small pieces. Whizz quickly, the mixture should look like breadcrumbs. Tip into a bowl, make a well in the middle and tip in the chilled water. Quickly, with cold hands, pull the flour into the water and pull into a ball of firm dough, do not ‘over work' it. Wrap the pastry in clingfilm and chill while the oven heats.
Remove the pastry from the fridge and rollout between two pieces of floured clingfilm to prevent it cracking or sticking. Using a rolling pin to pick it up, line a 20cm (8 inch) flan dish. If it tears, don't worry, just patch it together with your fingers, it is all going to be covered with the filling! Line the flan dish with tin foil and weight it down with dry beans or rice. Bake for 20 minutes then remove the foil and beans and continue to bake for a further 10 minutes or until the pastry is cooked and crisp.
While the pastry case is cooking mix 150g (6oz) of the sugar thoroughly with the cornflour in a pan, add the water and bring it to the boil over a low heat stirring continually. Cook the mixture until it thickens then remove it from the heat. When it has cooled slightly add the egg yolks and stir vigorously. Return to a very low heat and gradually add the butter in little bits and the lemon juice and zest. Taste to make sure that it is not too tart (or too sweet) for you and add more sugar or lemon accordingly. Spoon the curd into the pastry case.
Beat the two egg whites vigorously until they are stiff. Then add the remaining 100g (4oz) sugar and continue to whisk until the meringue is very stiff and shiny. Spoon over the top of the curd making sure that it is totally covered. Increase the heat of the oven to Gas Mark 5, 190ºC, fan 170ºC and bake for 8-10 minutes or until the top of the meringue is lightly browned.
Remove and cool before eating.
Cook's tip: to save time, you can also purchase ready-made gluten-free pastry in some Tesco stores. Click here to sign in to Tesco groceries to see if it's available in your local store.
See more Gluten-free recipes
If serving vulnerable groups, elderly people, babies, toddlers, pregnant women and people who are unwell, cook until the whites and yolks are solid.