Preheat the oven to 180°C (160°C fan assisted oven, 350°F, gas mark 4). Grease a 900g (2lb) loaf tin. Place the margarine and sugar into a bowl and beat together until pale and fluffy. Gradually beat in the eggs.
Sift in the flour and Baking Powder and add the ground almonds, poppy seeds and 1 ½ tsp (7.5ml) Lemon Extract. Gently mix all of the ingredients together until well combined.
Spoon the mixture into the prepared tin and smooth the top. Bake in the oven for 50-55 minutes until risen, firm to the touch and a skewer inserted into the centre comes out clean. Turn on to a wire rack to cool completely.
To decorate, sieve the icing sugar into a bowl. Add the remaining Lemon Extract (1 tsp) and mix in approx. 20ml (4 tsp) of warm water to make a smooth, soft icing. Spread over the top of the cake, allowing the icing to drizzle down the sides. Leave to set for a few minutes before slicing to serve.