Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Meanwhile, make the batter by combining the flour, cornflour and baking powder in a bowl. Whisk in enough beer to make a batter the consistency of single cream.
Dip the prawns into the batter, shaking off any excess, then deep-fry for 4-5 minutes, or until golden-brown and crisp.
Remove from the pan with a slotted spoon and set aside to drain on kitchen paper, then serve.