Remove cooked potatoes from oven. Increase oven temperature to 220c. Reheat bacon drippings in frying pan. Add garlic, thyme and peas and cook for 3 minutes. Return bacon to pan with sherry vinegar. Reduce heat to medium and cook for 2 minutes. Pour mixture over potatoes and dot with butter. Season well with salt and pepper. Return to oven for 5 minutes. Serve.