To make the caramel, place a large, heavy-based pan over a low heat.
Add 150g of the caster sugar and allow it to melt gradually.
Pour the caramel into a heatproof dish, roughly 25cm across and 5cm deep, or into 6 individual ramekins, then leave to set.
Pre-heat the oven to 150°C/130°C (fan) and begin making the custard.
Place the milk and cream in a large pan, along with the finely grated lemon zest.
As soon as it begins to simmer, remove from heat and infuse for 10 minutes.
Stir in the lemon juice.
Crack the eggs into a bowl, add yolks and remaining sugar.
Pour warm milk mixture into the eggs and sugar, then pour over the caramel.
Place in a roasting tray and bake in the oven until custard is set.
Cool, then refrigerate before serving with cream and cooked plums.
