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Heat the oven to 160C/fan 140C/gas 3.
Whizz 300g oatmeal biscuits to crumbs in a food processor (or put in a strong plastic bag and bash with a rolling pin).
Mix with 150g melted butter and press into the base and up the sides of a 22cm loose-based tart tin. Bake in the oven for 10 minutes. Remove and cool.
Put 3 medium egg yolks in a large bowl and whisk for a minute with electric beaters.
Add a can of condensed milk and whisk for 3 minutes, then add the finely grated zest and juice of 4 limes and whisk again for 3 minutes.
Pour the filling into the cooled base then put back in the oven for 15 minutes. Cool then chill for at least 3 hours or overnight if you like. When you are ready to serve, carefully remove the pie from the tin and put on a serving plate.
To decorate, softly whip together 300ml double cream and 1 tbsp icing sugar.
Dollop or pipe the cream onto the top of the pie and finish with extra lime zest.