OK. I think I have cracked it.
I had already been through the exact process Joha sent the link to, but then realised that Robin had told me something critical, that the device manager 'sees' the adaptor cable and allocates a Com port, this information is then requested by the Combro software.
So, while in device manager I plugged in just the serial / USB adaptor cable, sure enough it appeared in the list and was allocated Com3. I then plugged the Combro cable into the serial adaptor cable and then the cb-625 into the combro cable... and it all works.
So it looks like the order in which the hardware is plugged in makes a difference.
Of course, I now have to get it to work a second time.
"I'm your huckleberry, that's just my game"