Put the barley & water into a pan, bring to a boil & then cover & simmer for 30 minutes. Strain the liquid into a large jug. Peel the lemons very thinly without any white pith. Remove the pith from the lemons & slice the fruit. Put the fruit & lemon peel into the jug with the barley liquid. Stir in the sugar & let go cold. Strain into a clean jug & store in the refrigerator for up to 5 days.