Thanks,
My device is not enabled, but it is holding the RX line at 3V. This is upsetting the upload operation (the USB RX wants to operate at 5V). With a switch in RX, all works.
Also found that by limiting the drive from my device (in my case with a 560 ohm resistor in series), I can get both upload and receive working at the same time (no switch). NB - the Uno already has a 1K resistor between the micro pin and the upload chip