Brush a 23cm (9 inch) springform tin with a little vegetable oil. Crush the biscuits in a large plastic bag with a rolling pin until fine crumbs. Heat the butter and syrup in a saucepan until the butter has just melted. Take off the heat and stir in the biscuits until coated in the butter then tip into the tin and press into an even layer over the base.
Whip the cream in a large bowl until it forms soft swirls. Gradually whisk in the condensed milk, add the lemon rind then gradually whisk in the lemon juice until the mixture is smooth and thick. Fold in the blueberries. Pour into the tin, spread into any even layer then chill for 3-4 hours.
To make the sauce mix the cornflour and water together in the base of a small saucepan, add the sugar and blueberries and cook for 4-5 minutes over a gentle heat, stirring until the juices run from the blueberries and the sauce thickens. Leave to cool.
To serve, loosen the edge of the dessert, remove the tin sides and base and transfer to a serving plate, cut into wedges and serve with spoonfuls of the sauce.
*This pud is very rich and indulgent, so it shouldn't be eaten too often. But it's perfect for those special occasions.
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