Line a 18 x 23 cm roasting tin with foil, then lightly grease. Heat oven to 180C/160C fan/gas 4 and put a baking sheet in to heat up. Crush the biscuits until very fine.
Mix the crumbs and butter together well, then press into the bottom of the tin, squashing down firmly. Slide tin onto the baking sheet and bake for 20 mins until very dark golden all over.
Meanwhile, make the filling. Stir the flour and sugar together, make a well in the middle, then work in the eggs and yolk until smooth. Mix in the lime zest and juice. Pour the filling onto the hot base, return to the oven and turn the heat down to 160C/140C fan/gas 3. Bake for 20 mins or until set in the middle. Cool on a wire rack, then cut into bars.