Melt the margarine in a deep pan, then add the flour and cook for 3 minutes. Slowly add the milk and sugar and cook over a low heat for 8 minutes. Near the end of the cooking, add two pieces of mastica and mix. Remove from heat and place in blender whilst still hot, then blend. Dampen a large oven tray. Whilst the mixture is hot, spread onto the tray and chill for 4-5 hours in the fridge. Cut into small rectangles, the roll in coconut and place a piece of walnut on each place. Serve cold.