Hello, many thanks for replying on a Saturday. Everything works now. I think that there were several problems, and your reply fixed one problem before I'd encountered it.
I'm using Mac OSX and VirtualBox for development. 1) Sometimes the Mac fails to recognise the Shieldbuddy boards. This was my original problem. I don't know why this is. Unplugging and replugging the shieldbuddy often fixes this. 2) Virtualbox needs to be told to forward the Shieldbuddy USB device to the VM. But! By default also uses the serial number of the shieldbuddy in the USB filter. This meant I needed to add each shieldbuddy to the filter until I discovered that a blank serial number matches all. 3) I was using the DAS device scanner to make sure that the Shieldbuddy was recognised by Windows. Unfortunately, if I scan the device and then use the memory programmer, programming fails - every time. Unplugging and replugging the shieldbuddy allows the programmer to work, as does programming first and then using the scanner.