Prepare the pastry using the rubbing in method and bind together with the egg yolk and a little water if needed. Use to line a 24cm tart tin. Leave the excess pastry on the rim to overhang and do not cut off. Bake in the oven for 15 minutes blind at 190`c. When ready allow to cool down completely then cut off the extra pastry round the edge. Next prepare the filling by placing the caster sugar and 3 tbsp of water into a non-stick pan and cook to a rich caramel. Remove off the heat then stir in the butter followed by the cream. Continue to cook until the toffee is thick enough to leave a trail when mixed. Stir in the walnut halves then quickly pour the filling into the prepared pastry base. Level out with a spoon and continue to cook in the oven for 8 minutes or until it is bubbling. Cool for at least 15 minutes before serving.