Heat the oil in a medium saucepan, add the curry paste and cook for 1 minute. Stir in the coconut milk and fish sauce.
Add the lemon grass paste, lime leaves and green beans, cover and simmer for about 6 minutes until the beans are tender.
Stir in the noodles, crab, coriander and holy basil. Cook for 2 minutes, until the noodles and crab are heated through. Divide between 2 bowls and serve immediately.