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, and bring to the boil. Reduce the heat and simmer for 5 minutes.
Tip in the haricot beans and simmer for a further 5 minutes, then stir in half of the chopped parsley. Take off the heat and set aside while you cook the sausages.
Cook the sausages on the barbecue, or in a hot oven, 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 and let everyone help themselves.