Pare the zest from the lemons with a peeler, removing as little white pith as possible - cut away any pith you can from the strips. Juice the lemons and mix the juice, zest, sugar and raspberries with 1.2 litres of boiling water. Let cool, then sieve, pressing through juice with the back of a spoon. Add sugar to taste and chill in jugs. To serve, add a few lemon slices, raspberries and lots of ice.