You are seeing a 62-year-old male in A&E who has presented with a large painless rectal bleed. On questioning the patient also informs you that he experiences intermittent abdominal pain in the left lower quadrant, which is relieved by opening his bowels. The patient also informs you that he passes mucus out of his back passage occasionally. During examination, you note the presence of tenderness in the left lower quadrant. All observations are within normal limits and the patient feels systemically well. What is the most likely diagnosis?