Heat the oil in a pan over a medium heat and cook the onion for 5 minutes, adding the garlic for the last minute. Stir in the English mustard, treacle, tomato purée, sugar and chopped tomatoes, along with 400ml water. Bring to the boil, then reduce the heat and simmer for 5 minutes.
Tip in the haricot beans and simmer for a further 5 minutes, then stir in half the chopped parsley. Take off the heat and set aside.
Cook the sausages on the barbecue for 10-15 minutes, turning every few minutes, until cooked through and well-browned on the outside. Add the sausages to the pan of beans and warm on the barbecue. Top with the remaining parsley to serve.