Put the flour into a large bowl or food processor. Add the cubed butter, then work in with your fingertips or pulse to the texture of fine breadcrumbs.
Add small quantities of iced water (3-4 tbsp) until the dough just begins to come together. Knead briefly until smooth, then shape into a flat disc, wrap in cling film and chill for 30 minutes in the fridge.
On a lightly floured surface, roll out the pastry to about the thickness of a 50p piece and so it fits a 23cm x 2cm loose-bottomed, fluted round tart tin. Prick the base all over with a fork and chill in the fridge for a further 30 minutes.
Preheat the oven to 200°C/fan180°C/gas 6. Take the pastry case out of the fridge and line the tin with baking paper, I find it easier to scrunch the paper up and then flatten it back out, it really fits inside the case better that way. Place baking beans or rice on top of the baking paper then blind bake in the oven for 12 minutes.
While this is cooking, make the filling. Place the golden syrup and the treacle in a saucepan and warm gently until it becomes runny, add the breadcrumbs and lemon juice and give everything a good stir.
Take the pastry case out of the oven and reduce the oven temperature to 180°C/fan160°C/gas 4.
Remove the baking paper and baking beans from the pastry case and pour in the filling, bake for 20-25 minutes until just set.
Serve warm with clotted cream or custard.