Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease a 33cm x 22cm rectangular cake tin and line the base and sides with baking paper.
Mix the oil, cake mix, water and eggs gently together in a bowl, then whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy. Whisk in the lemon zest. Pour into the prepared tin and gently level the surface. Bake for 25-27 minutes until risen and golden and a skewer inserted into the cake comes out clean. Remove from the oven and cool in the tin for 10 minutes.
With the handle of a wooden spoon, poke holes at 1-2cm intervals almost to the bottom of the cake, wiping the spoon handle occasionally to reduce sticking.
To make the topping, stir the condensed milk, cream, lime juice and zest together in a bowl – the mixture will gradually thicken. Spread two-thirds of the mixture all over the cake and use the back of a spoon to spread evenly over the surface, working back and forth to fill the holes. Spread the rest of the mixture over the top of the cake and swirl with a palette knife. Chill in the fridge for 2 hours.
Carefully remove the cake from the tin using the baking paper. Slice into 15 servings and decorate each with lemon and lime slices, sliced strawberries and mint leaves.