Gradually add the egg and water mixture, 1 teaspoon at a time, and using a fork mix together until it makes a smooth ball with no crumbs falling away. If still crumbly, add in a little water, 1 teaspoon at a time, but be careful not to make the mix wet or sticky. Gently roll into a ball, wrap in clingfilm, and place in the fridge for at least 30 minutes until cold.