1. In a large mixing bowl, mix 250g crushed digestive biscuits with 125g melted butter.

2. Put the biscuit mixture into a glass. Press into the base and place in fridge.

3. Meanwhile, zest 4 limes into a small bowl. Roll each zested lime on a flat surface to help release as much juice as possible, then cut each one in half and juice, adding the juice to the bowl with the zest.

4. Put the tin condensed milk into a bowl and add 4 large egg yolks. Using an electric whisk, beat together for 1 minute. Add the lime zest and juice and beat for a further minute.

5. Pour into the glass and chill in the fridge to set completely.

6. In a bowl, whip 200ml double cream with 2 tbsp icing sugar to soft peaks. Dollop the cream over the key lime pie. Sprinkle over a little more lime zest to serve.