Mix the flour and sugar in a bowl, adding a pinch of salt if you like. Cut the butter into small cubes and rub into the flour until it resembles coarse breadcrumbs.
Stir in 1 tbsp water or more to bring the dough together to form a ball. Chill in the fridge for at least 30 minutes.
Roll out the pastry to line a 30 by 20 cm tart tin, pushing the pastry well into the sides. Chill for 30 mins then bake in a preheated oven at 190 c Gas 5 for 25 minutes until pale golden brown. Remove from the oven and allow to cool.
Cut the apricots in half and remove the stones. Lay them neatly in the tart case and return to the oven to bake for another 15-20 minutes depending how ripe the fruit is.
Brush with the apricot jam and return to the oven for 2 minutes. Serve warm or cooled.