Lemon Ice cream (good at Christmas)

This is an old family recipe that my sister Jane reminded me of


Lemon ice cream
Ingredients:  Makes a Ben and Gerry's size tub, double the ingredients for larger quantities
4 free-range eggs, yolks only
100g/3½oz golden caster sugar
1 tsp cornflour, optional
300ml/½ pint double cream
300ml/½ pint full-fat milk
3 lemons scrub the skin

First, separate the eggs, placing the yolks in a large bowl. (You will not need the whites for this recipe. You can use them to make meringues.)
Add the sugar to the egg yolks and whisk until pale and thick. 
Add the cornflour (if using) and whisk well to incorporate into the egg yolks. 
Put the cream and milk into a medium saucepan.
Grate the rind of both lemons and add to the milk and cream

Heat the cream and milk until just below boiling. Slowly pour the hot cream and milk onto the eggs and sugar, whisking as you go. 
Sieve the custard into a clean pan, and set it over a very low heat. Stir the custard constantly with a wooden spoon, until it is steaming and has thickened slightly. 

The custard is ready when you can draw a clear line through it on the back of the wooden spoon. This can take up to 10 minutes.
Allow the custard to cool so you can dip your finger in without pain and add the juice of the 3 lemons stir in quickly
Tip the custard into a large, shallow, freezer-proof container, and allow to cool to room temperature (you can speed this up by sitting the tub of custard in a large bowl of iced water). 
Once at room temperature, place a lid on the custard and chill in the fridge overnight.
Once chilled, transfer the custard to the freezer and take it out every hour, for three hours, to whisk it with an electric handheld whisk. This will disperse the ice crystals and keep it smooth. 
Then leave the ice-cream in the freezer for a final freeze, until it is solid.
Remove the ice cream from the freezer 10 minutes before serving, so that it’s easy to scoop.