Grate the rind of the lemons finely and place in the ovenproof bowl or top of the double boiler with the juice.
Add the sugar and butter and stir with a wooden spoon until the butter has melted and the sugar dissolved.
Add the eggs, slowly beating them into the mixture. Use a whisk if you prefer.
Now keep stirring slowly until the mixture starts to thicken. Don't be tempted to turn up the heat to speed things up or the mixture will curdle. It does take a while, possibly 30 minutes and will be the consistency of a custard when cooked. It thickens more as it cools.
Once the curd has thickened remove from the heat.
Pour into warmed jars. Seal the lids as you would with jam and when cold best keep it in the fridge or a cool place.