Place the raspberries in a medium saucepan over a medium heat with 300ml water. Use a speed peeler to strip the zest from the lemons into the pan. Bring to a gentle simmer, then cook for 10 to 15 minutes, stirring regularly, until the fruit has broken down. (Don’t let it boil.) Line a fine sieve with muslin over a large bowl (if you don’t have muslin, you can use a sheet of high-quality kitchen paper) and strain the mixture into a large bowl. Pour the liquid back into the pan, add the sugar and squeeze in the juice of both lemons. Turn the heat on to medium-low and stir regularly until all of the sugar has dissolved. Pour into a 1.2-litre sterilised bottle or jar and drink diluted with water, soda, cava or Prosecco.