Singapore chilli prawns with crisp noodles

Name: Singapore chilli prawns with crisp noodles
Ingredients: 50g rice vermicelli noodles; vegetable oil for deep-frying; 200g fine green beans, topped but not tailed; 1kg fresh, unpeeled raw prawns, about 15; 2 garlic cloves, finely; 5cm knob ginger, peeled and cut into matchsticks; 1 long, red chilli, finely sliced; 1 long green chilli, finely sliced; 2 tbsp sweet chilli sauce; 4 tbsp tomato sauce; 250ml hot chicken stock; ½ tsp salt; ½ tsp sugar; 2 tsp cornflour, mixed with 2 tbsp cold water