In a saucepan, melt honey with the margarine. Set aside to cool. Mix in flour, bicarb, sugar and water. Knead until smooth dough has formed. Divide into 3 parts. Roll out each part on lightly floured surface.
Preheat the oven to 180 C / Gas 4. Grease a 23x33cm baking tin.
Bake each piece of pastry for 15 to 20 minutes until golden brown. Remove from the oven and set aside.
Combine milk with semolina and bring to the boil. Cook over medium high heat for 30 minutes, stirring occasionally. Allow to cool.
Cream butter, sugar and vanilla extract until light and fluffy. Gradually mix in cooled the semolina mixture. Divide in half.
Spread 1/2 filling over first piece of pastry, cover with another pastry sheet, spread remaining filling and place the last piece of pastry on top.
For the icing:
Combine all ingredients, cook over low heat until smooth. Spread on top of the cake. Chill overnight.