In a saucepan, put 1 1/4 cups water with sugar, rhubarb & lemon juice. Cook over low heat , stirring until sugar has dissolved, then increase the heat a little; simmer 5 minutes. Remove from heat; leave the rhubarb to infuse for 10 minutes, then pass through a sieve, reserving the juice & pulp separately (the pulp can be used in the truffles that follow).