Preheat the oven to 200°C, fan 180°C, gas 6. Put the sausages in a large roasting dish with 1 teaspoon of the oil, the piri piri seasoning, garlic, green pepper and round tomatoes. Combine and season with freshly ground black pepper.
Put the potatoes in another large roasting dish. Mix with the remaining oil and season with freshly ground black pepper.
Place both roasting dishes in the oven and cook for 20 minutes.
In a small bowl, mix together the vinegar, Worcestershire sauce, tomato ketchup and 200ml boiling water. Pour over the sausages after they've been in the oven for 20 minutes, and add the tomatoes on the vine. Sprinkle the thyme leaves over both the sausages and the potatoes, and cook for a further 15 minutes.
Serve the sausages and their sauce with the roasted potatoes and tomatoes on the vine.