Add the pasta to salted boiling water and cook until al dente.
Keeping them separate, pat the prawns and calamari dry to remove excess moisture. Season both with a pinch of salt and pepper.
Add a drizzle of oil to a large pan over high heat. Once hot, space out the prawns and leave to fry for 1 minute until light golden on one side. Flip them over, cook for another minute, then remove from the pan. Top up with oil if needed, then space out the calamari and fry for 1 minute until it begins to firm up and take on some colour. Toss a few times during. Remove and place to one side, then lower the pan heat to medium.
Melt 1 tbsp of the butter, then add the garlic, chilli flakes and black pepper and fry for 30 seconds or so (be careful it doesn't burn). Pour in the wine and bring to a rapid simmer.
Immediately add the mussels to the wine and leave to cook until they start to open up, shaking the pan now and then to encourage them. Remove the mussels as and when they open and place on a plate to one side. You'll want to reduce the wine for around 4 minutes in total, so if the mussels are all cooked before that, continue simmering the wine to reduce it down (the harsh smell of alcohol should vanish).
Lower the heat back to medium and melt in the rest of the butter. Add the tomatoes, roasted red pepper and a good pinch of salt and fry for a couple of minutes, stirring as you go.
At this point, the pasta should be al dente, so turn the pan heat to low and use tongs to transfer the pasta straight from the pot into the pan, embracing any excess pasta water that comes with it. Give it a toss, then add the parsley (save some to serve) and lemon zest and continue tossing until the liquid thickens into a light, glossy sauce that clings to the pasta. The sauce shouldn't be watery at all.
Toss in the prawns and calamari, then add in the mussels, bringing any resting juices along the way. If the pasta dries out at any point, just toss in some leftover pasta water to loosen it up.
Check for seasoning and adjust (generously) if needed, then serve up with extra parsley and enjoy!