A 45-year-old man presents after four days of worsening right lower quadrant pain, fever and anorexia. Examination finds a tender, relatively fixed mass in the right iliac fossa, but there is no generalised peritonism and he is haemodynamically stable. CT shows an inflammatory appendiceal mass measuring 6 cm, with surrounding phlegmon but no free gas or drainable collection. Which one of the following is the most appropriate next step?