Spray a medium saucepan with low-calorie cooking spray and place over a medium heat.
Add the onion and fry for 5 minutes until slightly golden.
Add the garlic and chilli, and fry for a further 5 minutes, stirring.
Add the frozen seafood and cook for 8-10 minutes over a medium heat (or according to packet instructions), stirring well.
Add the fish stock, cayenne pepper, sweetener, tomato puree, and white wine vinegar, and season well with salt and pepper.
Simmer, uncovered, for 20-25 minutes. Stir in the chopped parsley.
While the fish is simmering, cook the pasta in a large pan of boiling water for 9-12 minutes or according to the packet instructions, until just tender. Drain well and remove from the heat.
Add the seafood sauce to the drained pasta and toss well.
Serve at once and garnish with a sprig of flat-leafed parsley, if you like.