A 79 year old woman is seen in A&E with a 2 day history of headaches and vomiting as her symptoms are becoming increasingly worse. Past medical history includes severe bilateral knee osteoarthritis for which she takes Co-codamol. The vision in her left eye is blurry and she is seeing coloured haloes. On examination there is a minimally reactive mid-dilated left pupil.
What is the most likely diagnosis?