For the prawns, place the vegetable oil into a deep, heavy-bottomed saucepan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the 150g/5Â½oz self-raising flour and sparkling water into a bowl. Season with salt and freshly ground black pepper and mix to make a batter the consistency of double cream.
Place the 50g/1Â¾oz plain flour into a shallow dish. Roll the prawns in the plain flour, shake off any excess and dip into the batter. Carefully place the prawns into the hot oil and deep fry for 6-8 minutes, or until golden-brown. Remove from the oil with a slotted spoon and drain on kitchen paper.
For the pesto, place all the pesto ingredients into a small food processor and blend until smooth.
For the saffron mayonnaise, place the egg and egg yolk, vinegar, mustard and garlic into a bowl. Season with salt and freshly ground black pepper and mix well.
Pour in the vegetable oil in a very slow trickle, whisking vigorously until all the oil has been incorporated and the mixture is thick.
Stir in the chopped spring onions, the saffron, along with its soaking liquid, and the parsley and taste to check the seasoning.
To serve, place the deep-fried prawns onto a large plate with the pesto and saffron mayonnaise in small bowls alongside and the lemon wedges, to garnish.