Preheat the oven to 325°F (160°C).
Boil the kettle.
Put the eggs, sugar, lemon juice and zest and the orange juice and zest in a large mixing bowl.
Beat until well combined but not too bubbly.
Add the double (heavy) cream and beat in. Again, be careful not to make the mixture too bubbly.
Set the mixture aside to rest while you prepare the Bain Marie.
Sit 4 or 6 heat proof ramekins or bowls in a large roasting pan or pans.
Being very careful not to splash any water into the ramekins, pour the water from the kettle into the larger pan(s) so that it comes half way up the ramekins.
Pour the custard carefully into the ramekins or bowls.
Using oven gloves, transfer the roasting pan(s) to the oven, being careful not to splash water into the custards on on yourself.
Bake for 20 to 25 minutes until the custard firms up a bit and the tops just begin to take on a golden tinge. Watch them carefully towards the end of the cooking time or they will burn.
When the custards are cooked, remove the roasting pans from the oven, being careful not to splash any hot water on yourself or in the custards. Place the roasting pans on a heat proof surface.
Using oven gloves, remove the ramekins or bowls from the roasting pans, dry the bottoms and place on a heat proof surface to cool to room temperature.
When cool, garnish with orange rind and chocolate curls if desired.
Serve at room temperature within a couple of hours if possible.