Thank you all! I'm afraid I can't take the credit for today's idea. Apart from the fact it was in the park, this scenario actually happened a couple of weeks ago with a patient I was looking after in the hospital.
Bunch of mates playing with machetes, one got hurt, came into hospital, operated on, was told he needed to stay in overnight for IV antibiotics, got stroppy, and despite explanations etc decided to discharge himself against medical advice. I probably couldn't have made this up myself! 