Tip the caster sugar into a saucepan and add 250ml water. Bring slowly to the boil, stirring to dissolve the sugar. Boil for 2 minutes and then remove from the heat.
Finely grate the zest from 3 of the lemons and add to the sugar syrup. Cut all of the lemons in half and squeeze the juice – you should end up with about 225ml/8fl oz of juice. Add to the syrup, stir well and leave to cool.
Cover and chill the lemon syrup for at least 2 hours until thoroughly cold.
Add another 100ml/3½fl oz of cold water to the sorbet mixture and churn in an ice cream machine according to the manufacturers instructions.
Alternatively pour the mixture into a plastic freezer box, cover with a lid and freeze for 1 hour. Remove from the freezer and whisk the half-frozen sorbet to break up the ice crystals, and return to the freezer for a further 40 minutes.
Continue whisking the sorbet mixture every 30–40 minutes, until it has completely frozen.
When making sorbet without using an ice cream machine you might find that the ice crystals are slightly larger and the sorbet is not as smooth as you’d like. To rectify this, scoop the frozen sorbet into a food processor and whizz for 20 seconds until smooth and then return to the freezer until frozen again.