Roll out the pastry on a floured counter and, using your pie dish as a template, cut out a piece to generously fit the top. From the trimmings, cut a 3/4-inch wide strip (it can be in pieces) to fit around the rim of the dish. Dampen the edges and stick the strips in place around the rim. Mix the ham and tarragon into the chicken filling, then spoon this into the dish. Dampen the top of the pastry strip and place the pastry round on top. Trim the edges and press down gently. Decorate the top if you wish with pastry shapes made from the trimmings, then brush with the beaten egg and make a small slit in the top for a steam hole. Either freeze or cook within 4 hours (keep the pie in the fridge).