I mix lemon juice and icing sugar together, then let it warm slightly on top of the cooker so that the sugar dissolves. When the cake comes out of the oven I use a cake testing skewer (not a fat meat skewer) to make holes in the crust and pour over the icing very slowly so that as much of the icing as possible can be absorbed into the cake rather than run down the sides (I make my LDC in a loaf tin rather than as a tray bake).
I use granulated sugar and lemon juice. First I skewer the top also with a cake tester and then spoon the mix of granulated sugar and lemon juice over. The granulated sugar gives it a slightly crunchy topping.