Dump the spaghetti in plenty of boiling water.
Cook past the al dente stage, drain, rinse in cold water and drain again.
In a wok or frying pan, melt the butter.
Saute the sliced onion and chopped garlic until softened.
Add the sliced sausages and bell pepper, and cook, stirring, for a minute.
Sprinkle in the sugar and pour in the ketchup, Worcester sauce, milk and half of the Parmesan. Cook, stirring, until the sauce us bubbly.
Add the cooked spaghetti to the sauce and toss to coat the noodles evenly.
Sprinkle in the rest of the Parmesan and toss once more.
Taste the ketchup spaghetti. Depending on the saltiness of the sausages, you may want to add more salt. Or you may want to sprinkle in a bit more sugar for balance.