Put the shallots, lime juice, vinegar, fish sauce and caster sugar in a bowl and mix until the sugar dissolves, then add the chilli and leave to macerate.
Meanwhile, cook the rice noodles, rinse under cold water and drain really well. Brush the skin of the sea bass fillets with oil then grill skin side up for 4-5 minutes.
Toss the noodles in the shallot mix with the cucumber and herbs, saving some of the herbs for the top. Divide the noodles between 2 plates and put a fish fillet, skin side-up, on top of each with wedges of lime to squeeze over.