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Heat oven to 180C/fan 160C/gas 4. Tip the biscuits into a food processor and blitz to crumbs. Add the butter and pulse to combine. Tip the mix into a fluted rectangular tart tin, about 10 x 34cm (or 20cm round tin) and press into the base and up the sides right to the edge. Bake for 15 mins until crisp.
While the base is baking, tip the egg yolks, sugar, and lime and lemon zests into a bowl and beat with an electric whisk until doubled in volume. Pour in the condensed milk, beat until combined, then add the citrus juices.
Pour the mix into the tart case and bake for 20 mins until just set with a slight wobble in the centre. Leave to set completely, then remove from the tin, cool and chill. Serve in slices topped with thin lime slices, if you like.