Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
Make the sponge by mixing the butter and caster sugar together in a large bowl, ideally using an electric mixer
Add the eggs and the orange, lime and lemon zest, and whisk until fully incorporated
Add the self raising flour and whisk in gently until you can't see any flour anymore
Divide the mixture between the two cake tins, using scales for accuracy if you like
Bake for 30-35 minutes or until a thin skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or on cooling racks
While it is baking, make the Pimms syrup. Put the Pimms, citrus juice and sugar in a pan, heat on a low heat until the sugar has dissolved. Then boil it for 5 minutes, stirring frequently. Pour into a bowl and leave to cool
If the cakes have domed on top, level them off with a cake leveller
Then brush or drizzle the Pimms syrup onto each sponge
To make the filling, mix together the double cream, vanilla extract and icing sugar with an electric mixer until thick. Then fold in the mascarpone cheese
Put one of the sponges on your chosen cake stand or serving plate and pipe or spread some of the filling over it
Spread the strawberry jam over the cream filling
Top with the second sponge and pipe larger rosettes of the filling around the edge of the cake, then spread the rest into the middle
Arrange the strawberries, orange slices, cucumber rolls or slices and fresh mint leaves on top of the cake. To make the cucumber rolls, use a vegetable peeler to peel long strips of cucumber, then roll them up
Serve immediately, leftovers will keep for 1-2 days in the fridge