Thanks Lee.
I connected a monitor, and briefly got a PAW splash screen with what looked like a progress bar at a low state of completion, then things stopped and the screen blanked. I left it for several more minutes and nothing happened.
On checking the SD card, it had started unpacking the files, as there was more on it than after a fresh download. I've subsequently tried two different freshly formatted cards with clean downloads and I can't get the box to do anything now, even after removing and reseating the bridge.
On power up, it now consistently flashes the green LEDs by the power socket and on the GPS chip once, the bridge green LED flashes three times, and the red power LED stays on, but absolutely nothing happens.