Pour some warm water into a roasting tin, and have a clean tea-towel laid out flat on the work-surface. Put 1 spring roll wrapper into the water for 30 seconds, then remove and lay it flat on the tea-towel. Turn it over to remove any excess water.
Arrange 4 prawn halves (pink side down) in the centre of the third of the wrapper nearest to you. Top with some lettuce, cucumber, spring onion, peanuts and coriander and finish off with 2 more prawn halves (pink side facing upwards).
Fold the edge of the wrapper nearest to you over the filling, and bring the edges over that, then tightly roll away from you right to the end to form a parcel (it is important to be firm and roll it really tight). Finally, take a sharp knife and cut the rolls in half diagonally – about 1cm in from each end. Repeat with the other spring rolls (they can be made several hours ahead; keep them covered with damp kitchen paper in the fridge).
For the dipping sauce, mix the first 4 ingredients together, sprinkle with spring onions and chilli and serve with the spring rolls.