Put the blackberries, water, sugar, mint and lime in a cocktail shaker and mash with the end of a rolling pin. Add the rum and some ice and shake well. Put the extra mint leaves and some ice into a glass and sieve in the cocktail adding a splash of soda water. Garnish with lime wedges, mint and blackberries by scattering on cocktail.
Recipe by appetite magazine at https://www.appetitemag.co.uk/blackberry-mint-mojito/