Rub the margarine into gluten free plain white flour until it resembles breadcrumbs.
Add the water to form soft dough cover and leave to rest for 30 mins.
Place the dough between two pieces of greaseproof paper and roll it out until it is large enough to line a 23cm flan case.
Chill the pastry in the flan case for 20 minutes. Prick the pastry case base, line with greaseproof paper and fill with baking beans (or rice if you haven’t the latter). Bake for 12 minutes.
Remove the paper and beans (rice) and bake until golden approx. 6-8 minutes.
In a saucepan, combine the lemon rind and juice with 1 cup of water, ½ cup of sugar or fruit sugar, and the margarine. Bring to the boil.
Meanwhile, dissolve the cornflour in the remaining 1 tbsp water. Add the egg yolks to the cornflour mix. Beat into the lemon mixture, return to the boil and whisk until thick, about 5 minutes. Leave to cool.
For the meringue, beat the egg whites with the salt and cream of tartar until stiff peaks form. Add the remaining sugar and beat until glossy.
Spoon the cool lemon mixture into the pastry case. Spoon the meringue on top of the lemon mix.
Bake until golden approx. 12-15 mins.