Ah, I thought, I have an SD card interface for my Windoze computer, in the shape of my Kobo bookreader. All appeared to work a treat - The image file wrote "successfully" to the SD card plugged into the Kobo, with the Kobo connected via USB to the PC. Wouldn't boot.. So tried a different card. Same thing. Re-downloaded the image file and image writing software and tried again. Same result. Tried a different power supply. Tried a different monitor. Different cables. Tried the analogue video socket. Same result. Finally, tried different PC with integral SC card support. Works perfectly!
I still can't believe that they didn't put some code in the firmware to do something to show that the Pi is "working", when powered up with nothing but the power connected and no SD card installed. Just flashing the LEDs in a pretty pattern would do.