Heat the oil in a pan, add the onion and bacon and fry over medium heat for 5 minutes, stirring now and then. Add the tomatoes, sugar, vinegar and 100 ml water, and simmer for 20 minutes.
Stir in the drained beans and continue to simmer for a few more minutes to warm the beans through. Add a dash of hot sauce if you like, and the chopped parsley.
Whisk two of the eggs, the harissa and seasoning in a shallow bowl. Add 4 of the crumpets and immerse on both sides; the crumpet will soak up the eggs like a sponge.
Heat half the butter and a drizzle of oil in a pan, add the four egg-soaked crumpets and fry for around 2 minutes each side over a medium heat, until golden and crisp.
Remove and keep warm while you repeat with the remaining eggs and crumpets. Spoon the bacon beans on top to serve.