Place the fish sauce in a large bowl. Add the lime juice, sugar, crushed red pepper flakes, and ¼ tsp salt and stir until the sugar has dissolved. Add the shallot and carrot and stir to combine.
Rinse the squid under cold running water and remove any guts or plasticy quills. Slice the bodies crosswise into very thin rings and cut any larger tentacles into pieces (leave smaller ones whole).
Half fill a saucepan with water and bring to the boil. Add 2 tbsp lemon juice and 1 tsp salt. Place squid in the water in batches and cook until opaque (30 secs-1 min). Drain and place on paper towel to cool and dry.
Place cooled squid in the bowl with the dressing, stir and refrigerate for 1 hour. Serve with mixed salad leaves and the herbs.
Recipe by appetite magazine at https://www.appetitemag.co.uk/thai-squid-salad/