Preheat the oven to 180°C/Fan 160°C/Gas 4. Grease a 30 x 23cm tray bake tin and line with non-stick baking paper.
Measure the baking spread, caster sugar, flour, eggs, baking powder, lemon curd and half of the lemon and lime zests into a mixing bowl. Beat together using an electric mixer until light and fluffy.
Turn the mixture into the tin and level the top. Bake in the preheated oven for about 30–35 minutes, or until well risen and the top of the sponge springs back when lightly pressed with a finger. Leave to cool in the tin.
To make the icing, measure the icing sugar and lemon and lime juices into a bowl. Whisk until smooth, then spread over the cake. Sprinkle with the remaining lemon and lime zest before cutting into 16 pieces.