Put the chicken in a bowl and add the lemon juice, chilli powder and a pinch of salt. Mix, cover with cling film and transfer to the fridge to marinate for 20 minutes.
Preheat a deep-fat fryer to 180C/350F. (CAUTION: Hot oil can be dangerous. Do not leave unattended).
For the beer batter, mix the flour, bicarbonate of soda and a pinch of salt in a bowl. Stir in the cold beer and enough sparkling water to make a smooth batter (you may not need all the water). Leave to stand for 10 minutes.
Whisk the egg white in a clean, dry glass bowl, until soft peaks form when the whisk is removed. Gently fold the egg white into the batter.
For the chicken, put the flour on a plate. Toss the chicken in the flour until coated, shake off any excess and dip in the batter.
Deep-fry the chicken for 6-8 minutes, or until crisp, golden-brown and cooked through with no sign of pink in the juices when you cut the thickest strip in half. Remove using a slotted spoon and drain on kitchen paper. Season with salt and serve.