Catastrophic brake failure. No fluid left in the reservoir. Driving without brakes is an interesting and engaging experience and one that nobody should have to suffer for too long. I've since topped up the brake fluid and there is of course air and now a little pressure in the system, for the mean time. The car goes to the shop on Tuesday.
Just before the brakes died it had a pressure wash and a valet on Friday, shame it's not really up to being driven.