Heat oven to 200C/180C fan/gas 6. Take one small sheet of foil, about A4 size, and put one fish fillet on top. Season the fish, then drizzle with half of the lemon juice and zest, half the sliced chillies and half of the basil.
Halve 4 of the tomatoes and put these around the fish. Place another sheet of foil on top and fold the edges together to seal. Repeat with the other piece of fish and transfer to a baking sheet. Cook for 15-18 mins until the bag has puffed up (or cook on the barbecue).
While the fish is cooking pour 100ml of boiling water over the couscous, cover, then leave to swell for 5 mins. Chop the rest of the tomatoes and mix with the couscous, balsamic, cucumber, the remaining basil, lemon juice and zest and olives. Season and serve alongside the fish with the lemon wedges on the side for squeezing over.