Prepare the raspberry jelly and leave to set in a round bowl in the fridge for 1 hour.
Puree raspberries and icing sugar in a food processor, then strain to remove seeds.
Simmer milk with half the caster sugar while whisking egg yolks with the remaining sugar until pale.
Whisk the warm milk into the egg mixture until combined.
Cook the custard until thickened to coat a spoon, then dissolve soaked gelatine into the hot mixture.
Combine the custard with the raspberry puree and chill for 30 minutes.
Preheat the oven to 190°C and line two 25x38cm Swiss roll tins with parchment.
Whisk eggs, sugar, and vanilla until fluffy, then fold in the flour by hand.
Spread the mixture into tins and bake for 10-12 minutes until springy.
Turn sponges out onto damp, sugar-dusted tea towels and remove parchment.
Spread jam on the sponges and roll them up tightly using the towels.
Oil a large bowl and line it with three layers of cling film.
Cut Swiss rolls into 1cm slices and line the bowl without leaving gaps.
Place the set jelly into the center of the lined bowl.
Fold whipped double cream into the raspberry custard, pour over the jelly, and chill for 4 hours.
Upturn onto a platter, remove film, decorate with strawberries and mint, and serve.