Preheat the oven to 180 degrees centigrade. Line a 20cm square tin with baking paper
In a large bowl, place all the ingredients except the margarine and boiling water. Place the margarine on top of the dry ingredients, then pour over the boiling water. This will melt the margarine into the batter as you whisk together all the ingredients until well combined. The mixture will be very runny
If using, stir in the crystalised ginger
Pour into the lined tin and bake for 20-25 minutes until golden on top and a knife comes out clean
Cool in the tin before moving to a wire rack
Whilst the cake is cooling, whisk together the margarine, cream cheese, icing sugar and lemon juice until you have a thick and smooth icing
Once the cake is cool, spread the icing all over the traybake and sprinkle with extra chopped crystalised ginger