OP here. Thanks for all the comments overnight.
@Brefugee, of course there are lots of reasons why people might not attend. But this hospital, which I've got to know pretty well in the last 10 years, with familial experience of A&E, cardiology, orthopaedic trauma, oncology, radiotherapy and dermatology in the last 12 months, is actually very very good and quietly efficient. It serves a low-income city plus a big, low-income rural catchment including a large elderly population.
The administration is generally good. We had two letters, three text messages and a phone to confirm everything, plus a courier delivery of the medication needed in preparation. This clinic has been planned since mid-October, before the mail strike.
It was day case surgery and all patients were told to arrive by 7.30am.
It was cold, but dry and not foggy, the roads were clear and mostly gritted.
The condition being treated is irksome and causes anxiety but it's not life-threatening. It mainly affects people who have undergone and survived quite major heart issues, so while one, even two, could have died waiting the chances of two-thirds doing so are statistically unlikely. Not impossible, but unlikely.
Of course, there are always going to be people with mental health issues and disorganised home lives: I am not sure how the NHS solves all the social ills.