Place flour in a food processor. Add the rest of the pastry ingredients and pulse until the ingredients come together in a ball. Divide dough into two pieces, one about 2/3 and the other 1/3 of total pastry. Shape each into a rough disc. Wrap in cling film and refrigerate for at least 3 hours.
Meanwhile have your blackberry marmalade at room temperature. To make the marmalade put the blackberry, the sugar and the orange juice in a pot on a medium heat. When it starts to boil, put it on a low flame and cook it for 40 minutes. Use a mouli to sieve your marmalade and let it cool down.
Roll out the larger of the two discs of pastry and line a 24cm-wide tart case with sides at least 3-4cm high and a removable base. Trim edges, removing any excess pastry. Fill pastry case with marmalade. Roll out remaining pastry. Cut 8 x 1cm-wide strips and place them on the top of tart in a criss-cross pattern and trim any overhang.
Bake in a preheated 180C oven for 30-40 minutes till golden. Remove from oven and let it cool on a wire rack. Dust with icing sugar and slice to serve. Serves 8-10.