My Gran passed this on to all our family and we have enjoyed it for years.
3 oranges and 3 lemons
3 lb sugar
1 oz tartaric acid
1 oz citric acid
1 oz epsom salts
8 cups boiling water
(To update to metric 3 lb equals 1.39 kgs. 1 ounce equals 28 gms)
Put juice and rind in a bowl, add sugar, acids and epsom salts, then pour over boiling water.
Stir until sugar is dissolved, then bottle.
This tastes great with lemonade or just add water.