A very careful check of the system has revealed a deeper problem. My connections were correct but the ASCOM/USB hub had switched cameras from my 'main camera#2' (=Ultrastar) to the guide camera! By directly connecting the camera to the laptop and not via the hub, it correctly downloaded the proper camera.
To do this check I had to switch off the camera after disconnecting from SGP. When I re-connected the camera and went to download a focus image (just to test), SGP crashed again and subsequently refused to do anything. I had to restart the laptop. This is fully reproducible. If a camera connection fault is announced SGP hangs on the message ad infinitum and then goes white when any key is pressed. Every time - suggesting that there is no error trapping on the camera connection.
Lawrence