Place the plum halves in a medium saucepan with 100g of the sugar and the lemon juice. Cover and cook gently for 15 minutes until the sugar has dissolved and the plums are sitting in their juice, then remove the lid and increase the heat. Simmer the plums for a further 8–10 minutes until they are very soft and have collapsed in a thick syrup – similar to a fruit compote. Stir regularly as they cook so that they do not stick. Spoon into a well-buttered 2-litre ovenproof pie dish. Leave to cool for 30 minutes.
While the plums are cooling, separate three of the eggs and beat the three yolks with one whole egg in a large bowl until smooth. Reserve the whites. Stir in 25g of the remaining sugar along with the lemon zest, softened butter, breadcrumbs, milk and cream. Leave to stand for 15 minutes to allow the breadcrumbs to swell. Spoon the bread mixture gently on top of the cooled plums, spread evenly and allow to stand for a further 15 minutes. Meanwhile, preheat the oven to 160°C fan/180°C/Gas 4.
Bake the pudding on a sturdy baking tray in the centre of the oven for about 35–40 minutes until faintly browned around the edges and only just set – it should still wobble in the middle. It is very important that the bread filling remains very soft and giving as when cooked to ‘cakey’ the pud is not such a treat. Whisk the egg whites until stiff and gradually whisk in the remaining sugar until the meringue is thick and glossy. Beat in the vanilla extract at the end. Remove the dish from the oven, increase the oven temperature to 170°C fan/190°C/Gas 5 and pile the meringue on top. Return to the oven for a further 18–20 minutes until the meringue is set and lightly browned. Serve hot with single cream for pouring. Dive in.