Heat the vegetable oil in a large, heavy-based saucepan to 180° C.
Sift together the plain flour, the baking powder and cornflour in a large mixing bowl and add enough soda water to form a batter, whisking as you pour into the dry ingredients (don't worry too much about lumps in the batter, it should be 'just' mixed).
Dust a few chunks of apples for each fritter with a little plain flour. Drop into the batter then deep-fry, in batches, until golden brown in colour all over; usually 2-3 minutes. Remove and drain on kitchen paper. Arrange on a serving plate when ready to serve.