Place bacon in a large, deep frying pan and cook until evenly browned. Drain, reserving 2 tablespoons of pan drippings. Chop bacon and set aside in a large bowl.
Cook the onion and garlic in the reserved pan drippings until onion is tender; drain off excess fat and transfer to the bowl with the bacon.
Add the borlotti, cannellini, baked beans, kidney beans and chickpeas to the bowl. Stir in tomato sauce, treacle, brown sugar, Worcestershire sauce, mustard and black pepper. Mix well and transfer to a 23x30cm oven-proof dish.
Cover and bake for 1 hour. Serve warm or at room temperature.