Put the tea into a large heatproof bowl with the sugar. Pour over the boiling water and stir until the sugar has dissolved, then cover and leave to stand for 1 hour.

Strain the tea through a very fine sieve into a jug. Stir in the lemon juice, then pour into a 1 litre (2 pint) metal loaf tin or other freezerproof container. Cover tightly with foil and freeze in the freezer (or freezing compartment of refrigerator turned to its coldest setting) for 2 1/2 hours, or until half-frozen and slushy.

Remove from the freezer, turn into a bowl and mash with a fork to break up the ice crystals, then whisk briefly until smooth. Return to container, cover and freeze for a further 2 hours, or until firm.

In a clean, dry bowl, whisk egg whites until standing in soft peaks.

Remove the tea ice from freezer, turn into a large bowl and break up with a fork as before, mashing well. Whisk the ice until smooth, then slowly whisk in the egg whites. Return the mixture to the container, cover and freeze for a further 4 hours, until firm.

To serve: remove from the freezer and soften at room temperature for 20-30 minutes. Scoop or spoon into small serving dishes. Serve at once.