For the berry sauce: Place the frozen berries in a medium saucepan and heat for a few minutes until they soften, crushing them with the fork or spoon. Add the sugar, lemon zest and 1/4 cup water and cook until thickened and reduced, for a further 5 minutes. Set aside and cool.
For the layers: Whisk the cream cheese until soft and smooth in a mixing dish. Add the yoghurt a little at a time, beating well after each addition to prevent any lumps from forming. Continue until all the yoghurt is mixed in with the cream cheese.
In a deep trifle dish, start with a layer of the Swiss roll slices. Top with some custard. Next, add some of the yoghurt and cream cheese mix and drizzle some berry sauce around the edge, followed by a layer of raspberries and strawberries. Next, repeat the layering with the Swiss roll slices, custard, yoghurt and cream cheese mix and then the berry sauce and fresh mixed berries. Repeat the layering for the last time, ending with the strawberries and finally a drizzle of the sauce. Garnish with fresh mint.
Serve immediately or cover with cling wrap and keep in the fridge until needed.