In a heavy based lidded pan heat half the olive oil over a moderate heat. Add the onion and cook for a few minutes, stirring constantly. Add the garlic, season with salt and pepper and cook for a further 2 minutes.
Add the tomatoes and bring to the boil, reduce the heat to a simmer and cook with the lid on for 2 minutes, add the wine and then the mussels and clams. Cover and cook until the shellfish opens, this is the base of the sauce. Discard any mussels and clams that remain closed.
Meanwhile, in a frying pan put the heads and shells of the prawns and crush them with the back of a spoon, turn up the heat and let them roast a little moving once or twice, they will go pink and start to smell like barbeque prawns. When they have some colour add the vegetable oil and let it sizzle, move the pan off the heat and add half of the butter, again it should sizzle. Season with salt and pepper.
Strain off the liquid from the prawn shells and squeeze out all of the juice, this is your prawn flavoured butter.
For the spaghetti, bring a large saucepan of salted water to the boil and cook for 10-12 minutes, or until al dente.
In a separate pan put the remaining olive oil and butter. Add the prawns, scallops and white fish. Cook on one side and turn carefully then add the chillies and cook for 2 minutes. Add the prawn flavoured butter and then pour over the tomato sauce and let it bubble. Season with salt and pepper.
Drain the spaghetti and drop into the pan with the seafood and sauce. Add the parsley, give it a good mix and let it bubble. Divide between four bowls and serve.