In a medium saucepan, combine the water, tonic water, and caster sugar.
Finely grate the lemon zest and add to the sugar mixture, before placing a sieve over the saucepan and juicing in the lemons - this will ensure that no pips or pith find their way into your sorbet.
Add the bunch of mint, stalks and all.
Place the saucepan over a medium heat and bring to a momentary boil, then remove from the heat.
Clamp on a lid and leave to cool completely.
Once the sorbet mixture has cooled, use a sieve to filter out all the zest and mint leaves, then transfer into a freezer-safe container.
Add 4 tablespoons of limoncello and stir.
Freeze for at least 4 hours, or until completely crystallised.
Transfer the sorbet to a food processor, and pulse to break up the ice-crystals, then return everything to your container and re-freeze.
If you prefer a finer texture to your sorbet, once it has refrozen, repeat the step of using the food processor.
Alternatively, if you have an ice-cream maker at home, at the point you add the limoncello, churn according to the manufacturers instructions.