Preheat the oven to 180°C/gas mark 4. Grease and line three 20cm cake tins with greaseproof paper.
Place the softened butter and caster sugar in a large bowl and beat together for 5 minutes until pale and fluffy. Add the eggs one by one, beating well between each addition.
Gently sift in the flour, then fold through the milk. Divide the mixture between the three tins and smooth the surface. Cook for 20-30 minutes, until risen and golden and a skewer inserted in the middle comes out clean.
Meanwhile, place the sugar and Pimm's in a small saucepan over a low heat until the sugar has dissolved. Add the diced strawberries and bring to the boil over a high heat. Boil, stirring often, for 10-15 minutes, until reduced and very thickened. Transfer to a heatproof bowl to cool.
Once cooked, leave the cakes to cool in their tins for around 10 minutes, then transfer to wire racks to cool completely.
Once the cakes and jam have cooled completely, place the cream, Pimm's and icing sugar in a large mixing bowl and, using a handheld electric mixer on a high speed, whip for 3-4 minutes until thick and holding its shape.
To assemble, place one of the cakes on a large plate or cake stand and spread with half the jam. Top this with a third of the whipped cream. Place a second cake on top and cover this with another third of the cream and the rest of the jam. Finally, top with the final cake, then decorate with the rest of the cream, strawberries, cucumber slices, strawberries, orange segments and mint leaves.