Bartender's notes

The syrup is the secret, and it can be made up to a week ahead. Keep it in a clean bottle in the fridge, and use any leftover for pouring over ice cream, porridge or pancakes. Fresh or frozen blackberries both work, and frozen ones often give a deeper colour. Taste before you top up with soda: if the syrup is sweet, add a squeeze more lemon.

With 50 ml of gin this comes to around 2 UK units per glass, so serve with care. Making a jug? Mix the gin, syrup and lemon ahead and chill it, then pour over ice and top up with soda at the table so it stays fizzy. For an alcohol-free version, leave out the gin, use 20 ml of syrup and 15 ml of lemon juice, and top with 150 ml of soda or tonic.