Preheat the oven to 180°c / gas 4. Grease a cake tin and set aside.
With an electric whisk, beat together the margarine and sugar until pale and fluffy.
Add the vanilla essence to the lightly beaten egg and in another bowl add the baking powder to the flour.
Add third of the egg to the margarine mixture and beat together with a wooden spoon, then add a third of the flour and combine, repeat until all the cake ingredients are mixed together.
Add the lemon juice and two thirds of the blueberries and gently fold in.
Pour the mixture into the cake tin, scattering the remaining blueberries over the top.
Bake in the oven for 25mins or until the cake has risen, golden brown and springs backs when gently pressed in the centre.
Remove from the oven, loosen the edges with a knife and allow to cool for 5mins. Turn out onto a wire rack to cool completely. Serve when ready.