It turns out that the procedures for getting signed off sick from work in the UK have changed since the last time I had pituitary surgery. I didn't know this, and nor did most of the doctors, nurses, surgeons and assorted healthcare workers at the hospital, but apparently that these days the hospital can only sign you off sick for the period you were actually in the hospital. Your GP has to certify you for the rest of the time. Seems a bit random given that the consultants at the hospital probably have a much better idea of how much time off you need, but nevermind.
Anyway, this has resulted in me receiving from my surgeon a little certificate entitling me to a grand total of three days off work. Reason given: brain surgery.