Chris & Jared, thank you for your replies. I think that it is probably the USB to Serial converter via a USB hub that is creating the hiccups - that is a guess, but the Prolific driver seems to be a bit flakey, so I will point the finger at that. Anyway, the great thing is that ticking 'Trace' appears to have made it work and if it works again next time and the time after that and forever more, I will be a happy imager!
If it doesn't work next time or in the future, I will be back to ask for your further assistance no doubt!