Beat together the eggs and egg yolks in a large heat proof bowl
Add the lemon and lime juice, lemon zest, sugar and butter
Place the bowl on a saucepan of simmering water, ENSURE THE WATER DOES NOT TOUCH THE BOTTOM OF THE BOWL!
Stir the mixture with a wooden spoon until the butter has melted, then use a whisk to gently beat the mixture.
Have a little patience and continue to gently beat the mixture for about 5 minutes, the curd will gradually thicken. You are looking for the whisk to leave a light trail in the curds. The curds will continue to thicken whilst cooling.
Pour the curds into sterilised jars and cap.
Once the curds have cooled store in the fridge. Use within 4 weeks
To make this extra tangy replace one of the lemons with another lime. My wife uses the unused egg whites to make a meringue for some sort of desert - sometimes an Eton Mess.