Add the gelatine leaves, one at a time, to 150ml of cold water, and leave them to expand for 4 - 6 minutes.
Now pour 300ml water into a pan, along with the caster sugar and liquid glucose. Warm over a low heat and stir until the sugar has dissolved, then raise the temperature and boil until the syrup reaches 128°C (use a sugar thermometer to check).
While the syrup is heating, but before it reaches 128°C, whisk the egg whites and put to one side. Remove the gelatin leaves from the water and squeeze out any excess moisture.
Remove the syrup from the heat, and carefully add the squeezed-out gelatine leaves (be careful not to burn yourself!) and stir until they have fully dissolved. If you want to add flavour (e.g. a couple of drops of vanilla