Bring the water and rhubarb to the boil, simmer for approx. 2 mins. Turn off the heat, cover and leave on the hob to infuse for approx. 15 mins. Pour the liquid and rhubarb through a sieve, retaining the juice.
Bring the juice, sugar, lime juice and raspberry powder to the boil, stirring constantly. Reduce the heat, simmer for approx. 2 mins. While still boiling hot, ladle the syrup into the clean, warmed bottle and fill to just below the rim, seal immediately, leave to cool.
Divide 2 tbsp of rhubarb syrup, the limes and sugar into glasses, crush gently with a fork. Pour in the rum and water. Add the rhubarb, mint, pepper and ice cubes.