In a large jug, mix the juice and grated lemon zest with the honey, grated ginger and caster sugar.
Pour in 150ml of the soda water. Leave to infuse for 10 minutes.
Top up with the remaining soda water. Using a fine sieve or a piece of muslin, strain the mixture into another large jug, discarding the zest and ginger pulp.
Either decant the ginger beer into small bottles if taking on a picnic, or just leave in the jug (covered with clingfilm), and chill in the fridge for at least an hour before serving with ice.