Preheat oven to 180 C / Gas 4. Grease a 20x30cm baking dish. Place the drained peas into the freezer.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the pasta and return to the boil. Cook the pasta uncovered, stirring occasionally, until cooked through, but still firm to the bite, about 5 minutes. Drain well in a colander set in the sink.
In a large saucepan over medium heat, melt the butter and saute the celery and onion until the onion has started to turn translucent, about 3 minutes. Stir in the courgettes and cook until tender, another 5 to 8 minutes. Mix in the soup and milk, then fold in half the cheese; allow the mixture to stand for 5 minutes. Gently stir in the pasta, then the seafood sticks and finally the peas from the freezer. Freezing the peas helps keep them intact. Season to taste with salt and pepper and spread the mixture into the prepared baking dish. Top with the remaining Cheddar cheese.
Bake in preheated oven for 30 minutes; remove from oven and cover with foil; allow to stand for 10 to 15 minutes to set up.