Put in a pan with the sugar and 150ml water, then heat, stirring gently, until the juices run and the sugar has dissolved.
Remove from the heat, then spoon into a large bottle or jar.
Add the vodka and mix well. Seal, then leave for 4 weeks before drinking to let the flavours steep. For ice cold vodka, keep the bottle in the freezer (it won’t freeze solid) or simply decant into a carafe, if you like.