Preheat the oven to 180°C, fan 160°C, Gas Mark 4. Line the base and sides of a 30 x 20 x 5 cm/12 x 8 x 2 inch rectangular cake tin (or roasting tin with the same base measurement) with a large piece of non-stick baking paper, snip diagonally into the corners then press the paper into the tin so that the base and sides are lined and the paper stands a little above the top of the tin sides.
Cream the soft margarine and sugar together in a bowl or food processor until light and fluffy. Mix the flour and baking powder together then gradually mix in alternate spoonfuls of egg then flour until both have all been added and the mixture is smooth.
Stir in the lemon rind and lemon juice until the mixture is a soft dropping consistency. Spoon the cake mixture into the lined tin, or for a patriotic red, white and blue marble cake, spoon half the cake mixture into the lined tin. Then divide the remaining cake mixture between two bowls, colour one half red, the remainder blue. Add spoonfuls of the coloured mixtures to the lined tin then swirl together with a small knife. Bake for 40-45 minutes until the cake is well risen, golden brown and a skewer comes out cleanly when inserted into the centre of the cake. Leave to cool for a few minutes then invert the cake on to a wire rack, remove the tin and leave to cool.
To make the buttercream, beat the butter and one third of the icing sugar together in a bowl or food processor until smooth. Gradually beat in the remaining icing sugar and lemon juice until light and fluffy. Cover and keep at room temperature until ready to use. Peel the paper off the cake, turn the cake over and trim the top level if needed. Cut in half then sandwich back together with a thin layer of buttercream. Put top down on a large plate, tray or cake board.
Spread a very thin layer of buttercream over the outside of the cake to stick the crumbs in place then spread a thicker layer all over the cake and smooth with a palette knife, reserving a couple of tablespoons for piping later.
Knead the ready to roll icing to soften then roll out half into a long strip on a work surface lightly dusted with sifted icing sugar. Trim to make 2, 4 cm (11/2 inch) wide strips long enough to make a cross over the centre of the cake then trim the ends level with the edges of the cake. Mix the trimmings with the remaining icing then roll out and cut two long strips about 2.5 cm (1 inches) wide and long enough to go diagonally from corner to corner of the cake, trimming the strips to sit flush with the wider white cross.
Arrange the blueberries between the strips of white ready to roll icing, see step pic. Spoon the remaining buttercream into a paper or plastic piping bag, snip off the tip then pipe dots on to the diagonal white strips and arrange the raspberries in lines on top. Add dots of icing over the central cross and stick the whole strawberries in place. Chill in the fridge until ready to serve.
Tip: to help you get ahead, the undecorated cake can be frozen wrapped in foil, take out the morning of the party and allow to defrost at room temperature for 3-4 hours.