Rinse the beans and pick over. In a large heavy saucepan, soak the beans in water (covering beans by 5cm/2in) overnight.
Bring the soaked beans to a boil in the water, adding more to cover the beans by approximately 2.5-5cm/1-2 in. Skim off any foam from the surface and gently simmer beans, partially covered, over a low heat for about two hours, or until the beans are tender but not falling apart. Drain.
In a heavy skillet, cook the bacon over a moderate heat until golden and crisp. Reserve 1 tbsp of the rendered fat to be added to the beans at a later stage.
Heat the olive oil in a large heavy saucepan over a medium heat. Add the onion, pepper, garlic, oregano and bay leaves and cook until the vegetables begin to soften, about 5 minutes. Add three quarters of the beans to the pan and mash coarsely. Add the remaining beans, water and vinegar and simmer, stirring occasionally, until the mixture thickens and the flavours are blended, about 30 minutes.
Roughly chop the bacon and add to the mixture with the reserved fat. Mix in the sugar and butter and season to taste with salt and pepper.
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