First make the short crust pastry: measure the gluten free flour into a large bowl and rub in the butter with your fingertips until the mixture resembles fine breadcrumbs (alternatively, this can be done in a food processor).
Add about three tablespoons of cold water and mix to a firm dough, wrap in cling film and chill in the fridge for about 20 minutes.
Preheat the oven to 200C/400F/Gas 6 and put a heavy baking tray in the oven to heat up. Grease a deep 18cm/7in loose-bottomed fluted flan tin with lactose free butter. .
Roll the pastry out thinly on a lightly floured work surface and line the prepared flan tin with the pastry. Prick the base with a fork, to stop the base rising up during baking.
To make the filling, heat the syrup gently in a large pan but do not boil. Once melted, add the gluten free breadcrumbs, lemon juice and zest to the syrup. (You can add less lemon if you would prefer less citrus taste.) If the mixture looks runny, add a few more gluten free breadcrumbs.
Pour the syrup mixture into the lined tin and level the surface. Bake on the pre-heated baking tray in the hot oven for about 10 minutes until the pastry has started to colour, and then reduce the oven temperature to 180C/350F/Gas 4.
Bake for a further 25-30 minutes until the pastry is golden-brown and the filling set. Remove the tart from the oven and leave to firm up in the tin. Serve warm or cold. With lactose free cream or lactose free ice cream.