Zest all but 1 lemon, ensuring you avoid any bitter white pith. Place zest, sugar, glucose syrup and water in a pan. Bring to the simmer. Once sugar has dissolved, allow to simmer for a further 5 minutes then set aside and allow to cool.
Juice the lemons, you should get between 1.25 and 1.5 cups of lemon juice.
Add the sugar syrup to the lemon juice, bit by bit, tasting each time, until you are happy with the balance of acidity and sugar. Remember that once frozen, the flavour will be muted so it does need to be quite intense. Strain through a fine sieve.
Chill the mix completely before adding it to an ice cream churner and following the manufacturer's instructions.
Once the sorbet is churned and almost set, whisk an egg white until light peaks form. Fold through sorbet before placing it in the freezer to finish setting.
To serve, place sorbet in the fridge for 20 minutes to soften slightly. Grate remaining fresh lemon zest over the top.